Plain or flash coat for detective on street? (6)
I believe the answer is:
modest
'plain' is the definition.
('modest' can be a synonym of 'plain')
'flash coat for detective on street?' is the wordplay.
'flash' becomes 'mo' (I've seen this in other clues).
'coat for' suggests removing the centre (the letters which coat the word).
'on street?' becomes 'st' (I've seen this in other clues).
'detective' with its middle removed is 'de'.
'mo'+'de'+'st'='MODEST'
'or' acts as a link.
(Other definitions for modest that I've seen before include "Not vain, rather shy" , "Mussorgsky, nominally?" , "Unostentatious" , "Simple, humble" , "Reserved" .)