Fine deed, in truth (4)
I believe the answer is:
fact
'in truth' is the definition.
(I've seen this before)
'fine deed' is the wordplay.
'fine' becomes 'f' (abbreviation).
'deed' becomes 'act' (I've seen this before**).
'f'+'act'='FACT'
(Other definitions for fact that I've seen before include "Truth, reality" , "It's certainly not fiction" , "True statement" , "Certain thing" , "Not fake news!" .)