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 living bird", "World's largest bird", "Large flightless bird of Africa", "Long-necked bird", "one who ignores home truths?", "source of showy feathers", "Someone unprepared to face the facts", "one tries to hide in the sand", "Fast winger" and "A bird for the shirt co.".)