Prey on the bosses in difficulty (6)
I believe the answer is:
obsess
'prey on' is the definition.
(I've seen this before)
'bosses in difficulty' is the wordplay.
'in difficulty' indicates an anagram (I've seen this in other clues).
'bosses' is an anagram of 'OBSESS'.
'the' acts as a link.
(Other definitions for obsess that I've seen before include "Be constantly preoccupied" , "Occupy the thoughts persistently" , "Haunt - preoccupy" , "Plague" , "Preoccupy continually" .)