Small courtyard boy used for air force unit (8)
I believe the answer is:
squadron
'for air force unit' is the definition.
(I've seen this before)
'small courtyard boy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'courtyard' becomes 'quad' (I've seen this before).
'boy' becomes 'ron' (**).
's'+'quad'+'ron'='SQUADRON'
'used' is the link.
(Other definitions for squadron that I've seen before include "Unit of the armed forces" , "Air, navy, cavalry, group" , "Regiment division" , "Cavalry unit" , "Air force grouping" .)