Detective takes position in gap (8)
I believe the answer is:
distance
'gap' is the definition.
(thesaurus)
'detective takes position' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'takes' is a charade indicator (letters next to each other).
'position' becomes 'stance' (stance is a kind of position**).
'di'+'stance'='DISTANCE'
'in' acts as a link.
(Other definitions for distance that I've seen before include "to make big winning margin" , "Ascend it from one point to another" , "Full extent of fighting" , "See 3" , "Length of space between one place and another" .)