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 explain this - if you can you should give a lot more credence to this answer).
'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' removed 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" .)