Endless cricket initially boring as tennis, surprisingly (9)
I believe the answer is:
incessant
'endless' is the definition.
(I've seen this before)
'cricket initially boring as tennis surprisingly' is the wordplay.
'initially' indicates taking the first letters.
'boring' indicates putting letters inside (some letters must 'bore' a hole into other letters).
'surprisingly' indicates an anagram (a surprising ordering of the letters).
The first letter of 'cricket' is 'c'.
'as'+'tennis'='astennis'
'astennis' with letters rearranged gives 'inessant'.
'c' going inside 'inessant' is 'INCESSANT'.
(Other definitions for incessant that I've seen before include "Continuous" , "Instances being continually repeated" , "Endless" , "Since Stan was continual." , "Constant, unremitting" .)