Pressure initially seems to change unknown instrument (8)
I believe the answer is:
psaltery
'instrument' is the definition.
(type of musical instrument)
'pressure initially seems to change unknown' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'initially' suggests taking the first letters.
'to change' becomes 'alter' ('alter' can be a synonym of 'change').
'unknown' becomes 'y' (algebra).
The initial letter of 'seems' is 's'.
'p'+'s'+'alter'+'y'='PSALTERY'
(Other definitions for psaltery that I've seen before include "Dulcimer-like instrument" , "Musical instrument" , "string instrument of yore" , "plucky thing" , "Plucked instrument; play rest (anag.)" .)