Have another check-up, using extra time in relaxation (6)
I believe the answer is:
retest
'have another check-up' is the definition.
I can't judge whether this definition defines the answer.
'extra time in relaxation' is the wordplay.
'extra time' becomes 'et' (this might be a standard abbreviation which I don't know about).
'in' means one lot of letters goes inside another.
'relaxation' becomes 'rest' (synonyms).
'et' inserted within 'rest' is 'RETEST'.
'using' is the link.
Can you help me to learn more?
(Other definitions for retest that I've seen before include "Make a further assessment" , "Examine afresh" , "another audition!" , "Set another examination" , "further exam" .)