Sailor, Yankee with girl climbing on the bed? (7)
I believe the answer is:
abyssal
'on the bed?' is the definition.
(relating to the sea bed)
'sailor yankee with girl climbing' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' means one lot of letters go next to another.
'girl' becomes 'lass' (lass is a kind of girl).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'lass' back-to-front is 'ssal'.
'ab'+'y'+'ssal'='ABYSSAL'
(Other definitions for abyssal that I've seen before include "regarding ocean trenches?" , "Bottomless" , "Belonging to ocean depths" , "on the bed" .)