Cold current, harsh, in tank (7)
I believe the answer is:
cistern
'tank' is the definition.
(I know that cistern is a type of tank)
'cold current harsh' is the wordplay.
'cold' becomes 'C' (abbreviation, eg on taps).
'current' becomes 'I' (physics symbol).
'harsh' becomes 'stern' (I've seen this in another clue).
'c'+'i'+'stern'='CISTERN'
'in' is the link.
(Other definitions for cistern that I've seen before include "Water container" , "WC water tank" , "Tank, reservoir" , "Toilet tank [ital]" , "Tank - sac of body fluid" .)