Inexpensive high explosive used in a detonator (5)
I believe the answer is:
cheap
'inexpensive' is the definition.
(similar in meaning)
'high explosive used in a detonator' is the wordplay.
'high explosive' becomes 'he'.
'used in' indicates putting letters inside.
'a detonator' becomes 'cap' (synonyms).
'he' inserted into 'cap' is 'CHEAP'.
(Other definitions for cheap that I've seen before include "Inexpensive or tastelessy showy" , "Second-rate" , "Not expensive - trashy" , "good value" , "Economical - rubbishy" .)