Troops run shelters to follow drug dealer (8)
I believe the answer is:
retailer
'dealer' is the definition.
(I know that retailer is a type of dealer)
'troops run shelters to follow drug' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'run' becomes 'r' (cricket abbreviation).
'shelters' is an insertion indicator.
'to follow' becomes 'tail' (both can mean to chase or pursue).
'drug' becomes 'e' (E is another name for ecstasy).
're'+'r'='rer'
'tail'+'e'='taile'
'rer' going around 'taile' is 'RETAILER'.
(Other definitions for retailer that I've seen before include "Seller, shopkeeper" , "One who sells goods to the geneal public" , "Tradesman" , "Shop" , "One selling to public" .)