Sort of test coming up for boy (3)
I believe the answer is:
tom
'boy' is the definition.
'sort of test coming up' is the wordplay.
'sort of test' becomes 'MOT' (MOT test for a vehicle).
'coming up' is a reversal indicator.
'mot' in reverse letter order is 'TOM'.
'for' is the link.
(Other definitions for tom that I've seen before include "Male feline" , "- Lehrer, US humorist" , "Thumb" , "Boy running off with pig" , "Jerry's cartoon antagonist" .)