A hundred troops send mad ambassador to get drink (5,2,6)
I believe the answer is:
creme de menthe
'drink' is the definition.
(creme de menthe is a kind of drink)
'a hundred troops send mad ambassador' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'troops' becomes 'REME' (Royal Electrical and Mechanical Engineers corps).
'send mad' becomes 'dement' (make someone demented).
'ambassador' becomes 'he' (His or Her Excellency).
'c'+'reme'+'dement'+'he'='CREME DE MENTHE'
'to get' is the link.
(Other definitions for creme de menthe that I've seen before include "booze" , "Green-coloured liqueur" , "Green peppermint-flavoured liqueur" , "Sweet green or white liqueur" , "Mint flavoured liqueur" .)