To catch public transport, first brace for Metropolitan Line (7)
I believe the answer is:
trammel
'to catch' is the definition.
(trameling is a kind of catching)
'public transport first brace for metropolitan line' is the wordplay.
I cannot quite see how this works, but
'transport' could be 'tram' (method of transport) and 'tram' is located in the answer.
'line' could be 'el' (el is a kind of line) and 'el' is found in the answer.
A single letter 'm' remains 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 trammel that I've seen before include "Place limits on" , "Hinder - impede" , "Hamper, as drag-net does" , "Three-layered fishing net - melt ram (anag)" , "bar" .)