Pair on alert, frolicking in common (11)
I believe the answer is:
proletarian
'common' is the definition.
(thesaurus)
'pair on alert frolicking' is the wordplay.
'frolicking' is an anagram indicator.
'pair'+'on'+'alert'='paironalert'
'paironalert' with letters rearranged gives 'PROLETARIAN'.
'in' is the link.
(Other definitions for proletarian that I've seen before include "Relating to the workers" , "Ordinary worker (Marxism)" , "Working-class" , "Characteristic of the working class" .)