Spread boron out, say (5)
I believe the answer is:
utter
'say' is the definition.
(to utter something is to speak or say it)
'spread boron out' is the wordplay.
'spread' becomes 'butter' (butter is a spread).
'boron' becomes 'B' (synonyms).
'out' is a deletion indicator.
'butter' with 'b' taken away is 'UTTER'.
(Other definitions for utter that I've seen before include "Total < state" , "Make sounds (but not necessarily words)" , "Thoroughgoing" , "Speak; complete" , "Without a single qualification" .)