Police squad tailing conservative on crack (7)
I believe the answer is:
crevice
'crack' is the definition.
(I know that crack can be written as crevice)
'police squad tailing conservative on' is the wordplay.
'police squad' becomes 'vice' (I've seen this in another clue).
'tailing' is a charade indicator (letters next to each other) (to tail can mean to go after or follow).
'conservative' becomes 'C' (UK political abbreviation).
'on' becomes 're' ('on' can be mean 'with reference to').
'c'+'re'='cre'
'vice' after 'cre' is 'CREVICE'.
(Other definitions for crevice that I've seen before include "Split" , "Narrow rock fissure" , "Crack or fissure." , "Cleft, fissure" , "Opening" .)