Very short saying: time to go for length (5)
I believe the answer is:
molto
'very' is the definition.
(musical term)
'saying time to go for length' is the wordplay.
I cannot quite see how this works, but
'time' could be 't' (abbreviation) and 't' is present in the answer.
'length' could be 'l' (abbreviation in geometry) and 'l' is located in the answer.
The remaining letters 'moo' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'short' is the link.
(Other definitions for molto that I've seen before include "Very musical Italian" , "very familiar to players" .)