Rhenium circuits closer to diode deteriorate (7)
I believe the answer is:
relapse
'deteriorate' is the definition.
(as in a relapse of a medical condition)
'rhenium circuits closer to diode' is the wordplay.
'rhenium' becomes 'Re' (Re is the chemical symbol for rhenium).
'circuits' becomes 'laps' ('lap' can be a synonym of 'circuit').
'closer to' indicates one should take the final letters (the letter which ends or closes the word).
The last letter of 'diode' is 'e'.
're'+'laps'+'e'='RELAPSE'
(Other definitions for relapse that I've seen before include "causing setback to recovery" , "go back" , "Sink back into illness" , "Deterioration in health" , "Slip back to former state" .)