Hesitate to detain defender in custody (8)
I believe the answer is:
holdback
'defender in custody' is the definition.
The definition suggests a singular noun which matches the answer.
'hesitate to detain' is the wordplay.
'hesitate' becomes 'back' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'detain' becomes 'hold' (I've seen this in another clue).
'back' after 'hold' is 'HOLDBACK'.
Can you help me to learn more?
(Another definition for holdback that I've seen is " Check".)