Detectives turning up during lunch, say, for a health check (7)
I believe the answer is:
medical
'a health check' is the definition.
('medical' can be a synonym of 'health check')
'detectives turning up during lunch say' is the wordplay.
'detectives' becomes 'cid'.
'turning up' shows that the letters should be reversed in order.
'during' is an insertion indicator.
'lunch say' becomes 'meal' (lunch is a kind of meal).
'cid' in reverse letter order is 'dic'.
'dic' inserted within 'meal' is 'MEDICAL'.
'for' is the link.
(Other definitions for medical that I've seen before include "Of the science dealing with treatment of disease" , "Health check-up" , "examination carried out by a doctor" , "Relating to treatment rather than surgery" , "Claimed (anag.)" .)