Relaxation broken by current exam (5)
I believe the answer is:
resit
'exam' is the definition.
(as in resitting an exam)
'relaxation broken by current' is the wordplay.
'relaxation' becomes 'rest' (synonyms).
'broken by' means one lot of letters goes inside another (inserted letters break into the word).
'current' becomes 'I' (symbol for electric current).
'rest' enclosing 'i' is 'RESIT'.
(Other definitions for resit that I've seen before include "later exam" , "Retake exam" , "repeated test" , "second exam" , "second try at test" .)