Took on idiot beginning to undress by the sea (7)
I believe the answer is:
assumed
'took on' is the definition.
('assume' can be a synonym of 'take on')
'idiot beginning to undress by the sea' is the wordplay.
'idiot' becomes 'ass' (I've seen this before**).
'beginning to' says to take the initial letters.
'by' is a charade indicator (letters next to each other).
'the sea' becomes 'med' (abbreviation of Mediterranean).
The first letter of 'undress' is 'u'.
'ass'+'u'+'med'='ASSUMED'
(Other definitions for assumed that I've seen before include "False" , "Bogus" , "Took to be the case" , "Seized" , "Put it on - without being told?" .)