Ship containing room to raise onions (8)
I believe the answer is:
shallots
'onions' is the definition.
(shallot is a kind of onion)
'ship containing room to raise' is the wordplay.
'ship' becomes 'ss'.
'containing' is an insertion indicator.
'room' becomes 'hall' (hall is a kind of room**).
'raise' says the letters should be written backwards.
'to' in reverse letter order is 'ot'.
'hall'+'ot'='hallot'
'ss' placed around 'hallot' is 'SHALLOTS'.
(Other definitions for shallots that I've seen before include "Species of small onions" , "vegetables" , "Onion-like plants with clusters of bulbs" , "Small onion species" , "Onion-like plant with clusters of bulbs" .)