Damp, but not the second source of damp (4)
I believe the answer is:
mist
'damp' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?
'damp but not the second source' is the wordplay.
'damp' becomes 'moist' (synonyms).
'but not' is a deletion indicator.
'the second source' becomes 'o' (2nd letter of 'source').
'moist' with 'o' taken out is 'MIST'.
'of' is the link.
Can you help me to learn more?
(Other definitions for mist that I've seen before include "Cloud; thin fog" , "Mizzle; light rain" , "Obscure" , "Fret" , "Fine spray" .)