Mite or small bloodsucking fly (6)
I believe the answer is:
smidge
'mite' is the definition.
(both can mean a small amount of something)
'small bloodsucking fly' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bloodsucking fly' becomes 'midge' (midges suck blood).
's'+'midge'='SMIDGE'
'or' is the link.
(Other definitions for smidge that I've seen before include "very small part" , "Really tiny amount" .)