During course of stress, middle son's no good, then rallies (11)
I believe the answer is:
strengthens
'rallies' 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?
'during course of stress middle son's no good then' is the wordplay.
'during' means one lot of letters goes inside another.
'course of stress middle' becomes 'stre' (I can't explain this - if you can you should give a lot more credence to this answer).
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'stre'+'s'='stres'
'n'+'g'+'then'='ngthen'
'stres' placed around 'ngthen' is 'STRENGTHENS'.
Can you help me to learn more?
(Other definitions for strengthens that I've seen before include "Makes more robust" , "Fortifies" .)