Row engulfing golf opponent (5)
I believe the answer is:
tiger
'opponent' is the definition.
(I have seen 'Formidable opponent' mean 'tiger' so perhaps 'opponent' could also mean 'tiger')
'row engulfing golf' is the wordplay.
'row' becomes 'tier' (a tier is a row of things).
'engulfing' means one lot of letters goes inside another.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'tier' placed around 'g' is 'TIGER'.
(Other definitions for tiger that I've seen before include "One of the big cats" , "he got his stripes" , "Large wildcat poetically burning bright" , "Fierce person" , "Wildcat of Bengal" .)