Examination for some ambitious candidates (4)
I believe the answer is:
scan
'examination' is the definition.
(scan is a kind of examination)
'some ambitious candidates' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SCAN' can be found hidden inside 'ambitious candidates'.
'for' is the link.
(Other definitions for scan that I've seen before include "Study intently" , "Scrutinize" , "Read electronically" , "MRI or CAT say" , "Examine quickly" .)