Suppose one will receive letter from Greeks (5)
I believe the answer is:
opine
'suppose' is the definition.
(I know that suppose can be written as opine)
'one will receive letter from greeks' is the wordplay.
'will receive' indicates putting letters inside.
'letter from greeks' becomes 'pi' (letter from the Greek alphabet).
'one' going around 'pi' is 'OPINE'.
(Other definitions for opine that I've seen before include "Conjecture" , "Say what one thinks" , "Express or hold view" , "Consider" , "Believe" .)