Current heating unit with leads in assembly line (7)
I believe the answer is:
thermal
'current' is the definition.
(I've seen this before)
'heating unit with leads in assembly line' is the wordplay.
'heating unit' becomes 'therm' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'leads in' indicates taking the first letters (I've seen 'lead in' mean this).
The first letters of 'assembly line' is 'al'.
'therm'+'al'='THERMAL'
(Other definitions for thermal that I've seen before include "Warming" , "Related to heat" , "Concerned with high temperature" , "type of baths" , "Warm or hot - underwear?" .)