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 in another clue).
'to' is a charade indicator (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' means one lot of letters go next to another.
'gar'+'a'='gara'
'gist' put after 'gara' is 'GARAGIST'.

Can you help me to learn more?