Briefly adequate — though variable — increase in sound (7)
I believe the answer is:
amplify
'sound' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
'briefly adequate though variable increase' is the wordplay.
'briefly' means to remove the last letter.
'adequate' becomes 'ample' (I've seen this in another clue).
'though' becomes 'if' (I've seen this in other clues).
'variable increase' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'ample' with its last letter taken off is 'ampl'.
'ampl'+'if'+'y'='AMPLIFY'
'in' is the link.
Can you help me to learn more?
(Other definitions for amplify that I've seen before include "Make fuller, increase the strength of" , "Increase (a signal)" , "Expand" , "Make louder or fuller" , "Increase the strength of, say sound" .)