Slough House Joe getting one of the dogs switched on internally (5)
I believe the answer is:
river
'slough house joe getting one of the dogs' is the definition.
I can't tell whether this definition defines the answer.
'switched on internally' is the wordplay.
'switched' is a reversal indicator.
'on' becomes 're' ('on' can be mean 'with reference to').
'internally' becomes 'vir' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'vir'='revir'
'revir' in reverse letter order is 'RIVER'.
Can you help me to learn more?
(Other definitions for river that I've seen before include "Moon - - (song)" , "Thames is one" , "x or y, so to speak" , "eg Aire" , "It flows" .)