Nothing seen in Manila (3)
I believe the answer is:
nil
'nothing' is the definition.
('nil' can be a synonym of 'nothing')
'in manila' is the wordplay.
'in' says the answer is hidden in the clue.
'NIL' is hidden within 'manila'.
'seen' acts as a link.
(Other definitions for nil that I've seen before include "Score of nought" , "Latin zero" , "Nothing, no value" , "Nothing found in senility" , "Nothing, zilch" .)