vladamir,
1) If you can score a first at undergrad and, hopefully, masters level, you're almost certainly 'clever' enough. Completing a PhD thesis is usually, as the saying goes, about 10% inspiration and 90% perspiration. I know several excellent academics/scholars who never completed one and, conversely, plenty of dullards who sailed through in 'paint-by-numbers' mode...
2) I'd certainly echo all the cautionary notes being sounded here about
(a) being very clear as to why (or whether) you really want/need a PhD, and what it's likely to 'yield' for you in professional terms,
(b) being realistic about how you'll fit it in around your other commitments, work and personal. My own PhD gradually turned from being a labour of love to, well, basically just a labour. It took me nigh on ten years, alongside fulltime work and 'serial' parenthood...
and it was only sheer bloody-mindedness that kept me going. If you're only finishing up a current postgrad stint, you might consider taking a year out first to 'prep' yourself (define and research your topic, consider your choice of supervisor/institution, get as much of the groundwork done as possible, etc.) before registering, forking out fees and 'starting the clock'...
Have a look at wry take, it'll cheer you up or put you off, depending on your frame of mind...!