A head of Rugby understood slang (5)
I believe the answer is:
argot
'slang' is the definition.
('argot' can be a synonym of 'slang')
'a head of rugby understood' is the wordplay.
'head of' says to take the initial letters.
'understood' becomes 'got' (getting is a kind of understanding).
The first letter of 'rugby' is 'r'.
'a'+'r'+'got'='ARGOT'
(Other definitions for argot that I've seen before include "Specialised language" , "Particular group's jargon" , "secret language" , "Jargon or slang (of a group)" , "Groat (anag)" .)