Note the very small number (5)
I believe the answer is:
minim
'note' is the definition.
(type of musical note)
'very small number' is the wordplay.
'very small' becomes 'mini' (I've seen this in another clue).
'number' becomes 'm' (M is an example).
'mini'+'m'='MINIM'
'the' is the link.
(Other definitions for minim that I've seen before include "It's a downward stroke" , "Musical note - small amount of liquid" , "Half-semibreve" , "Half-note" , "Palindromic note" .)