Nocturnal tree-dwelling lemur half visible, certainly (3)
I believe the answer is:
aye
'certainly' is the definition.
(both can be used to answer in the affirmative)
'nocturnal tree-dwelling lemur half visible' is the wordplay.
'nocturnal tree-dwelling lemur' becomes 'aye-aye' (aye-ayes are nocturnal primates).
'half visible' suggests halving the letters.
'ayeaye' with half the letters taken is 'AYE'.
(Other definitions for aye that I've seen before include "Word of assent" , "Affirmative answer" , "Yes answer in voting" , "An affirmative vote" , "Yes; ever" .)