Will go on being soft in the head, you suppose (7)
I believe the answer is:
presume
'suppose' is the definition.
(I know that suppose is a more specific form of the action presume)
'will go on' is the wordplay.
'will' becomes 'p' (I can't explain this - if you can you should give a lot more credence to this answer).
'go on' becomes 'resume' (resuming is a kind of going on).
'p'+'resume'='PRESUME'
'being soft in the head you' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for presume that I've seen before include "Surmise" , "Take to be the case" , "Suppose; take liberties" , "Supreme (anag)" , "Suppose without proof" .)