Campaigner good about environment (first and second) (5)
I believe the answer is:
green
'campaigner' is the definition.
(green can mean an environmental campaigner)
'good about environment first and second' is the wordplay.
'good' becomes 'g' (abbreviation).
'about' becomes 're' ('re' means regarding or about).
'environment first' becomes 'e' (1st letter of 'environment').
'and second' becomes 'n' (2nd letter of 'and').
'g'+'re'+'e'+'n'='GREEN'
(Other definitions for green that I've seen before include "Ecological" , "The colour of growing grass" , "member of political party" , "Immature" , "like 29 [MARTIAN] , stereotypically" .)