Notice fissure at sea (6)
I believe the answer is:
adrift
'at sea' is the definition.
(I've seen this before)
'notice fissure' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'fissure' becomes 'rift' (rift is a kind of fissure).
'ad'+'rift'='ADRIFT'
(Other definitions for adrift that I've seen before include "'Floating without direction, not tied up (6)'" , "Going out of control" , "Disoriented" , "'Directionless, floating freely (6)'" , "Loose from moorings" .)