Plug gap at sea (6)
I believe the answer is:
adrift
'at sea' is the definition.
(I've seen this before)
'plug gap' is the wordplay.
'plug' becomes 'ad' (Advertisement plugs products).
'gap' becomes 'rift' (rift is a kind of gap).
'ad'+'rift'='ADRIFT'
(Other definitions for adrift that I've seen before include "Disoriented" , "like Mary Celeste?" , "Not anchored, floating freely" , "Unanchored at sea" , "'Directionless, floating freely (6)'" .)