Small bird, for instance, climbing twig (3,2)
I believe the answer is:
get it
'twig' is the definition.
(synonyms)
'small bird for instance climbing' is the wordplay.
'small bird' becomes 'tit' (I've seen this in another clue).
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'tit'+'eg'='titeg'
'titeg' back-to-front is 'GET IT?'.
(Other definitions for get it that I've seen before include "See the point" , "Be punished" , "twig?" , "Understand" .)