Number Ten's short memo (4)
I believe the answer is:
note
'memo' is the definition.
(I've seen this before)
'number ten's short' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'short' means to remove the last letter.
'ten' with its final letter taken off is 'te'.
'no'+'te'='NOTE'
(Other definitions for note that I've seen before include "Observation" , "Sound, record" , "Observe - musical symbol" , "Money-carrier > hit repeatedly by 'charmingly sweet' singer" , "Annotation; tone" .)