Old twitcher's first to capture small, colourful bird (7)
I believe the answer is:
Here is my best explanation:
'bird' is the definition.
(ostrich is a kind of bird)
'old twitcher's first to capture small colourful' is the subsidiary indication.
'first' indicates taking the first letters.
'to capture' indicates putting letters inside.
'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'.
(Other definitions for ostrich that I've seen before include "Largest extant bird", "The head-in-the-sand bird", "Emu-like bird", "Chorist (anag.)", "Flightless African bird", "Fast-running African flightless bird", "Largest living bird", "A bird for the shirt co.", "To Chris (anag)" and "One refusing to face facts; flightless African bird".)