New scout master's first convention (6)
I believe the answer is:
custom
'convention' is the definition.
(I've seen this in another clue)
'new scout master's' is the wordplay.
'new' is an anagram indicator.
'master' becomes 'M' (common abbreviation eg MA, Master of Art).
'scout' with letters rearranged gives 'custo'.
'custo'+'m'='CUSTOM'
'first' is the link.
(Other definitions for custom that I've seen before include "Convention" , "Regular trade" , "Habitual use" , "Traditional habit" , "Established usage" .)