Enthusiast taking rook for mate (6)
I believe the answer is:
friend
'mate' is the definition.
(thesaurus)
'enthusiast taking rook' is the wordplay.
'enthusiast' becomes 'fiend' (fiend is a kind of enthusiast).
'taking' means one lot of letters goes inside another.
'rook' becomes 'r' (abbreviation in chess).
'fiend' enclosing 'r' is 'FRIEND'.
'for' acts as a link.
(Other definitions for friend that I've seen before include "Comrade for finder" , "China" , "Ally, supporter" , "One in need is one indeed" , "Buddy, chum" .)