First to enter popular ship is firm (7)
I believe the answer is:
insists
'is firm' is the definition.
('insist' can be a synonym of 'be firm')
'first to enter popular ship' is the wordplay.
'first to' becomes 'ist' (I've seen this in other clues).
'enter' is an insertion indicator.
'popular' becomes 'in' ('in' can mean trendy).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'in'+'ss'='inss'
'ist' placed within 'inss' is 'INSISTS'.
(Other definitions for insists that I've seen before include "Demands firmly, not accepting refusal" , "Contends" , "Maintains firmly" , "Brooks no refusal" , "Maintains persistently" .)