A name’s held up and it’s “Jack” (6)
I believe the answer is:
seaman
'it's jack' is the definition.
(I've seen this in another clue)
'a name's held up and' is the wordplay.
'name's' becomes 'maes' ().
'held up' is a reversal indicator.
'and' becomes 'n' (common abbreviation for 'and').
'a'+'maes'='amaes'
'amaes' reversed gives 'seama'.
'seama'+'n'='SEAMAN'
(Other definitions for seaman that I've seen before include "Mariner" , "crew member" , "A manse (anag.)" , "Sailor" .)