It's fixed in advance for engineers in pressure group (6)
I believe the answer is:
preset
'it's fixed in advance' is the definition.
(I've seen this in another clue)
'engineers in pressure group' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'in' is an insertion indicator.
'pressure' becomes 'p' (scientific abbreviation).
'group' becomes 'set' (both can mean a collection).
'p'+'set'='pset'
're' placed inside 'pset' is 'PRESET'.
'for' is the link.
(Other definitions for preset that I've seen before include "Systems control" , "Determined beforehand" , "ready in advance" , "ready to go off when required" , "adjusted beforehand" .)