Man trap set in concrete by day (8)
I believe the answer is:
reginald
'man' is the definition.
(I've seen this before)
'trap set in concrete by day' is the wordplay.
'trap' becomes 'gin' (gin trap is a kind of animal trap).
'set in' is an insertion indicator.
'concrete' becomes 'real' (associated in meaning).
'by' is a charade indicator (letters next to each other).
'day' becomes 'd' (abbreviation for day).
'gin' inserted into 'real' is 'reginal'.
'reginal'+'d'='REGINALD'
(Other definitions for reginald that I've seen before include "Man's name" , "-- Jeeves, Bertie Wooster's valet" , "Man" .)