Barely adequate sailor in lapses at sea (8)
I believe the answer is:
passable
'barely adequate' is the definition.
(I've seen this before)
'sailor in lapses at sea' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' means one lot of letters goes inside another.
'at sea' is an anagram indicator.
'lapses' is an anagram of 'passle'.
'ab' inserted inside 'passle' is 'PASSABLE'.
(Other definitions for passable that I've seen before include "OK" , "Merely satisfactory" , "Good enough to be acceptable" , "Fairly good but not outstanding" , "examinations should be!" .)