Jack has trouble punching small men (6)
I believe the answer is:
sailor
'jack' is the definition.
(thesaurus)
'trouble punching small men' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'punching' means one lot of letters goes inside another (I've seen this in other clues).
'small' becomes 's' (abbreviation - e.g. clothes size).
'men' becomes 'or' (I've seen this in other clues).
's'+'or'='sor'
'ail' going inside 'sor' is 'SAILOR'.
'has' is the link.
(Other definitions for sailor that I've seen before include "Member of a ship's crew" , "Seaman" , "Sinbad, maybe" , "Old salt" , "10 [MARINER]" .)