A communication you'll understand when you get it (7)
I believe the answer is:
message
'a communication' is the definition.
(I know that communication can be written as message)
'you'll understand when you get it' is the wordplay.
I cannot really see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for message that I've seen before include "Errand" , "Spoken or written communication" , "Communication that is sent" , "bulletin" , "Missive" .)