Former partner pinches trains (9)
I believe the answer is:
expresses
'trains' is the definition.
(express is a kind of train)
'former partner pinches' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'partner pinches' becomes 'presses' (I can't justify this - if you can you should give a lot more credence to this answer).
'ex'+'presses'='EXPRESSES'
Can you help me to learn more?
(Other definitions for expresses that I've seen before include "States" , "Trains" , "voices" , "'Conveys, articulates (9)'" .)