Current regulator's letter from Greece intercepted by English immediately (8)
I believe the answer is:
rheostat
'current regulator's' is the definition.
(I've seen this in another clue)
'letter from greece intercepted by english immediately' is the wordplay.
'letter from greece' becomes 'rho' (letter from the Greek alphabet).
'intercepted by' indicates putting letters inside (I've seen this in other clues).
'english' becomes 'E' (abbreviation).
'immediately' becomes 'stat' (stat can informally mean at once).
'rho' going around 'e' is 'rheo'.
'rheo'+'stat'='RHEOSTAT'
(Other definitions for rheostat that I've seen before include "Variable resistor" , "Means of controlling resistance" , "Current-changing device" , "Variable resistance" , "Sort heat with Instrument that controls the current in an electrical circuit" .)