Old twitcher's first to capture small, colourful bird (7)
I believe the answer is:
ostrich
'bird' is the definition.
(ostrich is a kind of bird)
'old twitcher's first to capture small colourful' is the wordplay.
'first' indicates taking the first letters.
'to capture' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'colourful' becomes 'rich' ('rich' can be similar in meaning to 'colourful').
The first letters of 'old twitchers' is 'ot'.
'ot' placed around 's' is 'ost'.
'ost'+'rich'='OSTRICH'
(Other definitions for ostrich that I've seen before include "A bird for the shirt co." , "Rich sot seeks large bird" , "one tries to hide in the sand" , "Someone unprepared to face the facts" , "The biggest bird, can't fly" .)