Much to alter, perhaps, in a heartless guy (7)
I believe the answer is:
greatly
'much' is the definition.
(I've seen this before)
'alter perhaps in a heartless guy' is the wordplay.
'perhaps' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'a heartless' suggests removing the centre.
'guy' with its middle taken out is 'gy'.
'alter' anagrammed gives 'reatl'.
'reatl' put inside 'gy' is 'GREATLY'.
'to' acts as a link.
(Other definitions for greatly that I've seen before include "To a large extent" , "Very much" , "to a considerable extent" .)