Don't catch up with a convict at the back (3,6)

'the back' is the definition.
(thesaurus)

'don't catch up with a convict' is the wordplay.
'don't catch up' becomes 'behind' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'a convict' becomes 'lag' (slang term for a prisoner).
'behind' put after 'lag' is 'LAG BEHIND'.

'at' acts as a link.

Can you help me to learn more?