Single large unit (4)
I believe the answer is:
lone
'single' is the definition.
('lone' can be a synonym of 'single')
'large unit' is the wordplay.
'large' becomes 'l'.
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'l'+'one'='LONE'
(Other definitions for lone that I've seen before include "Solitary, sounds like advance" , "Companionless" , "Solitary or single" , "Unaccompanied" , "Sole" .)