Are they truly fatheaded deeds? (5)
I believe the answer is:
facts
'are they truly' is the definition.
The definition suggests an adverb but the answer is not.
'fatheaded deeds?' is the wordplay.
'fatheaded' becomes 'f' (this might be a standard abbreviation of which I'm unaware).
'deeds?' becomes 'acts' (deed is a kind of act).
'f'+'acts'='FACTS'
Can you help me to learn more?
(Other definitions for facts that I've seen before include "Dope" , "these cannot be disputed" , "Truths" , "Realities and certainties" , "hard information" .)