Concede that nobody can repress a cry of pain (5)
I believe the answer is:
allow
'concede' is the definition.
(synonyms)
'nobody can repress a cry of pain' is the wordplay.
'nobody can' becomes 'll' (I can't explain this - if you can you should give a lot more credence to this answer).
'repress' indicates putting letters inside.
'cry of pain' becomes 'ow' (exclamation of pain).
'a'+'ow'='aow'
'll' going into 'aow' is 'ALLOW'.
'that' is the link.
Can you help me to learn more?
(Other definitions for allow that I've seen before include "Permit or concede" , "praise no more" , "Tolerate" , "Grant" , "Admit as acceptable" .)