Keep saying is, but perhaps isn't, going out (6)
I believe the answer is:
insist
'keep saying is' is the definition.
I can't tell whether this defines the answer.
'perhaps isn't going out' is the wordplay.
'perhaps' becomes 'ins' (I can't justify this - if you can you should believe this answer much more).
'isn't' becomes 'is not'.
'going' becomes 'on' (both can mean functioning or activated).
'out' indicates named letters should be taken away.
'isnot' with 'on' taken out is 'ist'.
'ins'+'ist'='INSIST'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for insist that I've seen before include "Maintain strongly" , "Maintain persistently" , "Urge" , "Refuse to give way" , "Be firmly demanding" .)