Engineers on circuit secure drained sink again (7)
I believe the answer is:
'sink again' is the definition.
('lapse' can be a synonym for 'sink' so perhaps 'relapse' is OK for 'sink again')
'engineers on circuit secure drained' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'on circuit' becomes 'lap' (synonyms. I am not sure about the 'on' bit.).
'drained' suggests removing the centre.
'secure' with its centre removed is 'se'.
Can you help me to learn more?
(Other definitions for relapse that I've seen before include "Pleaser may slip to earlier state" , "Backsliding" , "Become ill again" , "Unexpected deterioration" , "Fall back into bad health" .)