Current mains supply, with no problem at night (8)
I believe the answer is:
insomnia
'problem at night' is the definition.
(I've seen this before)
'current mains supply with no' is the wordplay.
'current' becomes 'in' ('in' can mean current or trendy).
'supply with' is an anagram indicator (in a supple way).
'no' becomes 'O' (resembles 0 - 'no' can mean 'zero').
'mains' with its letters anagrammed with 'o' gives 'somnia'.
'in'+'somnia'='INSOMNIA'
(Other definitions for insomnia that I've seen before include "Habitual inability to sleep (8)" , "Sleep disorder" , "Not getting off" , "Condition affecting sleep" , "In restless state" .)