A head of a colliery in sports car, travelling (3,6)
I believe the answer is:
per capita
'a head' is the definition.
(I've seen this before)
'a colliery in sports car travelling' is the wordplay.
'colliery' becomes 'pit' (synonyms).
'in' indicates putting letters inside.
'sports' becomes 'pe' (Physical Education).
'car travelling' becomes 'rca' (I can't explain this - if you can you should believe this answer much more).
'a'+'pit'='apit'
'pe'+'rca'='perca'
'apit' inserted inside 'perca' is 'PER CAPITA'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for per capita that I've seen before include "For each person - in routine Latin phrase" , "A head" , "apiece" .)