Man might, having determination (9)
I believe the answer is:
willpower
'determination' is the definition.
(I've seen this before)
'man might' is the wordplay.
'man' becomes 'will' (short for William).
'might' becomes 'power' (being mighty).
'will'+'power'='WILLPOWER'
'having' is the link.
(Other definitions for willpower that I've seen before include "self-control" , "determination" , "Inner strength" .)