More powerful fighter with hard row (8)
I believe the answer is:
mightier
'more powerful' is the definition.
('mighty' can be similar in meaning to 'powerful')
'fighter with hard row' is the wordplay.
'fighter' becomes 'mig' (I've seen this in other clues).
'with' says to put letters next to each other.
'hard' becomes 'h'.
'row' becomes 'tier' (tier is a kind of row**).
'mig'+'h'+'tier'='MIGHTIER'
(Other definitions for mightier that I've seen before include "Having more power" , "Stronger" , "As the pen is relative to the sword" , "'Make thee - - yet' (Jerusalem)" , "More powerful" .)