Case for subtenant providing flat for firm (7)
I believe the answer is:
stiffen
'firm' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe you can see a link between them that I can't see?
'case for subtenant providing flat' is the wordplay.
'case for' says to hollow out the word (remove centre letters) (the letters which encase the others are used).
'providing' becomes 'if' (eg 'providing it's sunny' means 'if it's sunny').
'flat' becomes 'fen' (I can't justify this - if you can you should believe this answer much more).
'subtenant' with its middle taken out is 'st'.
'st'+'if'+'fen'='STIFFEN'
'for' is the link.
Can you help me to learn more?
(Other definitions for stiffen that I've seen before include "Make rigid or inflexible" , "Become more rigid" , "Make hard" , "Reinforce" , "Make unbending" .)