Gives support to his defence (5)
I believe the answer is:
backs
'gives' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Maybe you can see an association between them that I can't see?
'support to his defence' is the wordplay.
'support' becomes 'back' (to back is to support or endorse).
'to his defence' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'back'+'s'='BACKS'
Can you help me to learn more?
(Other definitions for backs that I've seen before include "Defensive players" , "Reverses" , "Gives support or one's blessing to" , "champions" , "delightful Cambridge area" .)