If not half a unit smaller (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this before)
'half a unit smaller' is the wordplay.
'half a' suggests halving the letters.
'smaller' becomes 'less' (less great in size).
'unit' with half the letters taken is 'un'.
'un'+'less'='UNLESS'
(Other definitions for unless that I've seen before include "Though" , "bar" , "Conditional on" , "Otherwise than if" , "Except, save" .)