Friend of Sarah decapitated (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'sarah decapitated' is the wordplay.
'sarah' becomes 'Sally' (Sally is a common pet form of Sarah).
'decapitated' means to remove the first letter.
'sally' with its initial letter taken away is 'ALLY'.
'of' is the link.
(Other definitions for ally that I've seen before include "Collaborator, affiliate" , "Accomplice" , "Crony" , "Supporter in a conflict" , "State formally cooperating with another" .)