Soft assonance heard in preparation (7)
I believe the answer is:
priming
'preparation' is the definition.
(priming is a kind of preparation)
'soft assonance heard in' is the wordplay.
I cannot quite understand how this works, but
'soft' could be 'p' (musical abbreviation for 'piano') and 'p' is found within the answer.
'in' is present in the answer.
This explanation may well be incorrect...
(Other definitions for priming that I've seen before include "Preparing for firing" , "Briefing" .)