General meaning to keep artillerymen back, and a good mechanic (8)

'good mechanic' is the definition.
The definition suggests a singular noun which matches the answer.

'general meaning to keep artillerymen back and a' is the wordplay.
'general meaning' becomes 'gist' (I've seen this before).
'to' says to put letters next to each other (I've seen this in other clues).
'keep artillerymen back' becomes 'gar' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'gar'+'a'='gara'
'gist' after 'gara' is 'GARAGIST'.

Can you help me to learn more?