Let's say it makes most masseuses mad! (6)
I believe the answer is:
assume
'let's' is the definition.
(thesaurus)
'say it makes most masseuses mad' is the wordplay.
I cannot really understand how this works, but
'say' could be 'as' (I've seen this in other clues) and 'as' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
The remaining letters 'sum' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for assume that I've seen before include "Take as true or take for granted" , "Believe" , "Accept as true without proof" , "Claim without justification" , "Adopt falsely" .)