Evoke birdsong, not one of the top three we hear (4,5)
I believe the answer is:
'evoke' is the definition.
'birdsong not one of the top three we hear' is the wordplay.
'birdsong' becomes 'call' (synonyms).
'not one of the top three' becomes 'fourth' (in fourth place).
'we hear' shows a homophone (sound like).
'fourth' sounds like 'forth'.
(Other definitions for call forth that I've seen before include "Summon" , "Evoke" .)