Go up into sanctum and fall (6)
I believe the answer is:
deepen
'fall' is the definition.
The definition and answer can be both related to change as well as being verbs in their base form.
Perhaps you can see a link between them that I can't see?
'go up into sanctum' is the wordplay.
'go' becomes 'pee' (both can mean to urinate).
'up' is a reversal indicator (in down clue: letters go upwards).
'into' indicates putting letters inside.
'sanctum' becomes 'den' (I've seen this in another clue).
'pee' backwards is 'eep'.
'eep' inserted inside 'den' is 'DEEPEN'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for deepen that I've seen before include "make room for more water" , "Get stronger" , "Make stronger" , "Make or become more profound" , "Increase in depth" .)