In a detonator, high explosive is not expensive (5)
I believe the answer is:
cheap
'not expensive' is the definition.
('cheap' is the opposite to 'expensive')
'in a detonator high explosive' is the wordplay.
'in' is an insertion indicator.
'a detonator' becomes 'cap' (synonyms).
'high explosive' becomes 'he'.
'cap' enclosing 'he' is 'CHEAP'.
'is' is the link.
(Other definitions for cheap that I've seen before include "Vulgar" , "Contemptible" , "Low in price or tastelessly showy" , "Tawdry - inexpensive" , "Base" .)