Careful in area certain to be overwhelmed by sea (8)
I believe the answer is:
measured
'careful' is the definition.
(I know that careful can be written as measured)
'area certain to be overwhelmed by sea' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'certain' becomes 'sure' (synonyms).
'to be overwhelmed by' indicates putting letters inside (inserted letters get absorbed or overwhelmed).
'sea' becomes 'med' (short for Mediterranean).
'a'+'sure'='asure'
'asure' going into 'med' is 'MEASURED'.
'in' acts as a link.
(Other definitions for measured that I've seen before include "Deliberate, restrained" , "Unhurried and with care and dignity" , "Calculated, quantified" , "Carefully considered - armed use (anag)" , "Regular in movement or carefully considered" .)