Express, let's suppose (3)
I believe the answer is:
say
This clue is a triple definition!
'express' is the first definition.
(I know that say is a more specific form of express)
'let's' is the second definition.
(thesaurus)
'suppose' is the third definition.
(I know that suppose can be written as say)
(Other definitions for say that I've seen before include "Speak, utter" , "Utter words" , "Utter, recite" , "SECOND PART OF 24" , "power" .)