Crossword Genius

Reactionaries one used to give protection on train? (3,5)

Ross

I believe the answer is:

old guard

I'm a little stuck... Click here to teach me more about this clue!

'reactionaries' is the definition.
(I've seen this before)

'protection on train?' is the wordplay.
'protection' becomes 'guard' (I've seen this before).
'on' means one lot of letters go next to another.
'train?' becomes 'old' (I can't explain this - if you can you should give a lot more credence to this answer).
'guard' after 'old' is 'OLD GUARD'.

'one used to give' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for old guard that I've seen before include "who may recall what railways used to be like" , "Most conservative members of a group - of sentries" , "Long-standing group members, set in their ways" , "Original or most conservative members of a group, like ancient sentry" , "Reactionaries" .)

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