Green light for the greatest drug employed as a last line of defence, in short (6)
I believe the answer is:
goalie
'employed as a last line of defence in short' is the definition.
I can't judge whether this definition defines the answer.
'green light for the greatest drug' is the wordplay.
'green light' becomes 'go' (I've seen this in other clues).
'for' says to put letters next to each other.
'the greatest' becomes 'ali' (film about Muhammad Ali).
'drug' becomes 'e' (e is another name for ecstasy).
'go'+'ali'+'e'='GOALIE'
Can you help me to learn more?
(Other definitions for goalie that I've seen before include "Soccer player who protects the target" , "On-field custodian" , "Football team member" , "Netkeeper, in short" , "Keeper (colloq.)" .)