Strengthen army corps having legal validity (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(reinforcing is a kind of strengthening)
'army corps having legal validity' is the wordplay.
I cannot really see how this works, but
'army' could be 'force' (I've seen this before) and 'force' is found within the answer.
'corps' could be 're' (Royal Engineers army regiment) and 're' is located in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for reinforce that I've seen before include "give added support" , "provide support" , "increase support" , "Make stronger" , "Strengthen or support with confrerie" .)