Shadow when area's illuminated round about (4)
I believe the answer is:
tail
'shadow' is the definition.
(both can mean to follow someone)
'area's illuminated round about' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'round' is an insertion indicator.
'about' says the letters should be written in reverse.
'a' inserted inside 'lit' is 'liat'.
'liat' reversed gives 'TAIL'.
'when' is the link.
(Other definitions for tail that I've seen before include "Hindmost part" , "Follow the caudal appendage" , "part of coat" , "Trail (behind comet)" , "Follow (a suspect)" .)