Starting to vent anger about a drawback for a slanting line? (7)

'slanting line?' is the definition.
'virgule' can be an answer for 'line?' (thesaurus). I'm not certain of the 'slanting' bit.

'starting to vent anger about a drawback for a' is the wordplay.
'starting to' suggests taking the first letters.
'anger' becomes 'ire' (ire is intense rage).
'about' means one lot of letters goes inside another.
'a drawback' says the letters should be written backwards.
'for a' becomes 'lug' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'vent' is 'v'.
'lug' in reverse letter order is 'gul'.
'ire' enclosing 'gul' is 'irgule'.
'v'+'irgule'='VIRGULE'

Can you help me to learn more?