Notes in a short manuscript (6)
I believe the answer is:
minims
'notes' is the definition.
(minim is a type of musical note)
'short manuscript' is the wordplay.
'short' becomes 'mini' (similar in meaning).
'manuscript' becomes 'ms' (abbreviation for manuscript).
'mini'+'ms'='MINIMS'
'in a' acts as a link.
(Other definitions for minims that I've seen before include "Musical half notes" , "Short musical notes" .)