Slightly spoil friend, drinking spirits (10)
I believe the answer is:
marginally
'slightly' is the definition.
(thesaurus)
'spoil friend drinking spirits' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'friend' becomes 'ally' (an ally is a friend or supporter).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'spirits' becomes 'gin' (gin is a kind of spirits).
'mar'+'ally'='marally'
'marally' enclosing 'gin' is 'MARGINALLY'.
(Other definitions for marginally that I've seen before include "Barely" , "in small measure" .)