Liquid or gel in a battery (11)
I believe the answer is:
electrolyte
'liquid or gel in a battery' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for electrolyte that I've seen before include "conductor" , "Liquid or gel containing ions - EEC telly rot (anag)" , "Fluid in a battery" , "Liquid or gel in a battery" .)