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 judge whether this definition defines the answer.
'perhaps isn't going out' is the wordplay.
'perhaps' becomes 'ins' (I am not sure about this - if you are sure 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' is the 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" .)