Man having to show zest (5)
I believe the answer is:
gusto
'zest' is the definition.
(synonyms)
'man having to' is the wordplay.
'man' becomes 'gus' ().
'having' is a charade indicator (letters next to each other).
'gus'+'to'='GUSTO'
'show' acts as a link.
(Other definitions for gusto that I've seen before include "Zest, exuberance" , "Spirit" , "Relish; vigour" , "Vigorous and enthusiastic enjoyment" , "Great enjoyment in doing something" .)