A small amount that is missing I guess (6)
I believe the answer is:
assume
'guess' is the definition.
(I've seen this before)
'a small amount that is missing i' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'amount' becomes 'sum' (synonyms).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'missing' indicates named letters should be taken away.
'ie' with 'i' taken out is 'e'.
'a'+'s'+'sum'+'e'='ASSUME'
(Other definitions for assume that I've seen before include "Take to be the case" , "Take as read" , "Take as true or take for granted" , "Take or put on" , "Take for granted or take upon oneself" .)