Flower — one held by riotous seamen (7)
I believe the answer is:
nemesia
'flower' is the definition.
(I've seen this before)
'one held by riotous seamen' is the wordplay.
'one' becomes 'i' (Roman numeral).
'held by' is an insertion indicator.
'riotous' is an anagram indicator.
'seamen' with letters rearranged gives 'nemesa'.
'i' going into 'nemesa' is 'NEMESIA'.
(Other definitions for nemesia that I've seen before include "Plant with colourful flowers, related to snapdragon" , "South African plant" , "Meanies (anag) - flower" , "Funnel-shaped flower" , "Colourful 13 across" .)