Note close friend's short, a tiny measure (6)
I believe the answer is:
micron
'a tiny measure' is the definition.
'micron' can be an answer for 'measure' (micron is a kind of measure). I'm unsure of the 'a tiny' bit.
'note close friend's short' is the wordplay.
'note' becomes 'mi' (as in 'do re mi').
'close friend' becomes 'crony' (I've seen this before).
'short' means to remove the last letter.
'crony' with its final letter taken away is 'cron'.
'mi'+'cron'='MICRON'
Can you help me to learn more?
(Other definitions for micron that I've seen before include "Micrometre" , "hardly any length" , "One-millionth of a metre" , "One millionth of 3 Across" , "Small length" .)