Source of gratuities at the race-course? (7)
I believe the answer is:
tipster
'source' is the definition.
(thesaurus)
'gratuities at the race-course?' is the wordplay.
'gratuities' becomes 'tips' ('tip' can be a synonym of 'gratuity').
'at' means one lot of letters go next to another.
'the race-course?' becomes 'ter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tips'+'ter'='TIPSTER'
'of' is the link.
Can you help me to learn more?
(Other definitions for tipster that I've seen before include "One who sells betting advice" , "One advising betters" , "Person offering advice" , "Adviser on betting" , "Giver of betting advice" .)