Transport is mostly broken (3)
I believe the answer is:
bus
'transport' is the definition.
(bus is a kind of transport)
'mostly broken' is the wordplay.
'mostly' means to remove the last letter.
'broken' becomes 'bust' (I've seen this before).
'bust' with its final letter taken off is 'BUS'.
'is' is the link.
(Other definitions for bus that I've seen before include "Means of public transport" , "Public transport vehicle" , "Large passenger vehicle - small business!" , "Coach" , "Something caught" .)