Loud noise, ultimately in head, may do so (6)
I believe the answer is:
deafen
'loud noise ultimately in head may do so' is the definition.
I can't tell whether this defines the answer.
'loud noise ultimately in head' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'ultimately' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'head' becomes 'dean' (dean is a kind of head).
The final letter of 'noise' is 'e'.
'f'+'e'='fe'
'fe' inserted into 'dean' is 'DEAFEN'.
Can you help me to learn more?
(Other definitions for deafen that I've seen before include "Need FA (anag.)" , "Drown out" , "Render unable to hear" , "Cause to lose sense of hearing" , "Cause loss of hearing to (someone)" .)