Smarter detective's breaking rank (6)
I believe the answer is:
tidier
'smarter' is the definition.
('tidy' can be similar in meaning to 'smart')
'detective's breaking rank' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'breaking' is an insertion indicator.
'rank' becomes 'tier' (tier is a kind of rank).
'di' placed into 'tier' is 'TIDIER'.
(Other definitions for tidier that I've seen before include "not so disorderly" , "more organised" , "'Neater, in better order (6)'" , "More neat and orderly" , "straightened up?" .)