Court activity, developing in earnest, ensnaring Earl at last (4,6)
I believe the answer is:
real tennis
'court activity' is the definition.
'real tennis' can be an answer for 'activity' (real tennis is a kind of activity). I'm not sure about the 'court' bit.
'developing in earnest ensnaring earl at last' is the wordplay.
'developing' indicates anagramming the letters.
'ensnaring' is an insertion indicator.
'at last' suggests the final letters.
The last letter of 'earl' is 'l'.
'in'+'earnest'='inearnest'
'inearnest' anagrammed gives 'reatennis'.
'reatennis' placed around 'l' is 'REAL TENNIS'.
Can you help me to learn more?
(Other definitions for real tennis that I've seen before include "Game played in court" , "Old sport" , "ancient game" , "In which court member served" , "Early form of Wimbledon's game" .)