Easy to lose head in defeat (4)
I believe the answer is:
lick
'defeat' is the definition.
(thesaurus)
'easy to lose head' is the wordplay.
'easy' becomes 'slick'.
'to lose head' means to remove the first letter.
'slick' with its initial letter removed is 'LICK'.
'in' is the link.
(Other definitions for lick that I've seen before include "Lap; defeat" , "Flog" , "Personally clean" , "Lap; trounce" , "Feel with the tongue" .)