Say 'let', audibly (5)
I believe the answer is:
aloud
'audibly' is the definition.
(I've seen this in another clue)
'say let' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'let' becomes 'allowed' (to allow something is to let it happen).
'allowed' is a homophone of 'ALOUD'.
(Other definitions for aloud that I've seen before include "Audibly" , "Using the voice, not silently" , "Take part in, link" , "For all to hear" , "in a normal voice" .)