Flourish in ship in open (7)
I believe the answer is:
blossom
'flourish' is the definition.
(I've seen this before)
'ship in open' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'in' is an insertion indicator.
'open' becomes 'bloom' (I've seen this before).
'ss' put within 'bloom' is 'BLOSSOM'.
'in' acts as a link.
(Other definitions for blossom that I've seen before include "Flowers on tree or bush" , "Produce flowers" , "Mature promisingly" , "attractive growth" , "Flowers (on tree, shrub)" .)