Occasionally call out men, big old boys (6)
I believe the answer is:
alumni
'old boys' is the definition.
(old boys can mean the alumni of a private school)
'occasionally call out men big' is the wordplay.
'occasionally' indicates alternate letters.
'call'+'out'+'men'+'big'='calloutmenbig'
The alternating letters of 'calloutmenbig' are 'ALUMNI'.
(Other definitions for alumni that I've seen before include "Schoolkids once" , "Past pupils or graduates" , "Past pupils, former students" , "we studied at a certain place" , "Graduates of a college" .)