Plant credit note on ship (5)
I believe the answer is:
cress
'plant' is the definition.
(I know that cress is a type of crucifer)
'credit note on ship' is the wordplay.
'credit' becomes 'cr' (abbreviation).
'note' becomes 'e' (musical note).
'on' means one lot of letters go next to another.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'cr'+'e'+'ss'='CRESS'
(Other definitions for cress that I've seen before include "Plant with pungent leaves used in salads" , "Cruciferous salad plant" , "Food item" , "Salad plant paired with mustard" , "Salad vegetable" .)