L-gap seen through on sailing ship (8)
I believe the answer is:
porthole
'through on sailing ship' is the definition.
'porthole' can be an answer for 'ship' (thesaurus). I'm not certain of the 'through on sailing' bit.
'l-gap' is the wordplay.
'l' becomes 'port' (I am not sure about this - if you are sure you should believe this answer much more).
'gap' becomes 'hole' (hole is a kind of gap).
'port'+'hole'='PORTHOLE'
'seen' acts as a link.
Can you help me to learn more?
(Other definitions for porthole that I've seen before include "firing through this" , "Ship's window" , "from which sea can be seen" , "Circular window in ship" , "Small watertight opening in ship's side" .)