Concern when oaf takes in nothing very well (7)
I believe the answer is:
lookout
'concern' is the definition.
('that's your lookout')
'oaf takes in nothing very well' is the wordplay.
'oaf' becomes 'lout' (synonyms).
'takes in' indicates putting letters inside.
'nothing' becomes 'O' (O resembles 0).
'very well' becomes 'OK' (synonyms).
'o'+'ok'='ook'
'lout' going around 'ook' is 'LOOKOUT'.
'when' is the link.
(Other definitions for lookout that I've seen before include "Watchman" , "Observation point" , "Watchtower" , "Outlook for sentry" , "Observer; concern" .)