Flowering maple left in U-boat at sea (8)
I believe the answer is:
abutilon
'flowering maple' is the definition.
(I've seen this before)
'left in u-boat at sea' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'u' is found in the answer.
'at' could be 'on' (location) and 'on' is found within the answer.
This explanation may well be incorrect...
(Other definitions for abutilon that I've seen before include "showy shrub" , "plant" , "Flowering maple" .)