Rely on installing hard drive (6)
I believe the answer is:
thrust
'drive' is the definition.
(I know that drive can be written as thrust)
'rely on installing hard' is the wordplay.
'rely on' becomes 'trust' (synonyms).
'installing' indicates putting letters inside (I've seen this in other clues).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'trust' placed around 'h' is 'THRUST'.
(Other definitions for thrust that I've seen before include "Push forcefully" , "Pierce" , "Force engine generates" , "Sudden lunge or attack" , "Push forward as with fencing sword" .)