Think about head of rival firm (6)
I believe the answer is:
sturdy
'firm' is the definition.
(thesaurus)
'think about head of rival' is the wordplay.
'think' becomes 'study' (studying is a kind of thinking).
'about' means one lot of letters goes inside another.
'head of rival' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'study' going around 'r' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Robust, strong" , "Well-built - strapping" , "Strong and sturdily built" , "Well-constructed" , "Uncompromising" .)