Charlie argued regularly with environmentalists' leader, becoming vulgar (5)
I believe the answer is:
crude
'vulgar' is the definition.
(synonyms)
'charlie argued regularly with environmentalists' leader' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'with' means one lot of letters go next to another.
'leader' says to take the initial letters.
The first letter of 'environmentalists' is 'e'.
The alternate letters of 'argued' are 'rud'.
'c'+'rud'+'e'='CRUDE'
'becoming' is the link.
(Other definitions for crude that I've seen before include "Raw, unrefined" , "Not expertly made" , "Rude; raw" , "Not refined, like oil" , "Not refined, oil say" .)