Let's suppose it's a 'put on' (6)
I believe the answer is:
assume
I believe this is a triple definition!
'let's' is the first definition.
(thesaurus)
'suppose' is the second definition.
(assume can mean to suppose or expect)
'it's a put on' is the third definition.
(I've seen this before)
(Other definitions for assume that I've seen before include "Take as true or take for granted" , "Take on, affect" , "Take for granted or take upon oneself" , "Claim without justification" , "Be arrogant" .)