Bossy, heartless, wild fellows (4)
I believe the answer is:
boys
'fellows' is the definition.
(I know this)
'bossy heartless wild' is the wordplay.
'heartless' means to remove the middle letters.
'wild' is an anagram indicator.
'bossy' with its centre removed is 'bosy'.
'bosy' anagrammed gives 'BOYS'.
(Other definitions for boys that I've seen before include "Young men" , "Beach --, group" , "They will be themselves, it's said" , "Male children" , "Lads" .)