Fellows taken in by heap of nonsense (6)
I believe the answer is:
piffle
'nonsense' is the definition.
(piffle is a kind of nonsense)
'fellows taken in by heap' is the wordplay.
'fellows' becomes 'ff'.
'taken in by' indicates putting letters inside.
'heap' becomes 'pile' ('pile' can be a synonym of 'heap').
'ff' inserted within 'pile' is 'PIFFLE'.
'of' is the link.
(Other definitions for piffle that I've seen before include "Hogwash" , "Rubbish, nonsense" , "Twaddle" , "Stuff and nonsense" , "Nonsense (colloq.)" .)