Because of hearty efforts? (3)
I believe the answer is:
for
'because' is the definition.
(I've seen this in other clues)
'of hearty efforts?' is the wordplay.
'of' indicates a hidden word.
'FOR' can be found hidden inside 'hearty efforts'.
(Other definitions for for that I've seen before include "In support of" , "On the side of" , "See 24" , "Favouring" , "On behalf of" .)