Playing tennis is, at first, terribly demanding (9)
I believe the answer is:
insistent
'demanding' is the definition.
(thesaurus)
'playing tennis is at first terribly' is the wordplay.
'playing' indicates anagramming the letters.
'at first' suggests taking the first letters.
The initial letter of 'terribly' is 't'.
'tennis'+'is'='tennisis'
'tennisis' with letters rearranged gives 'insisten'.
'insisten'+'t'='INSISTENT'
(Other definitions for insistent that I've seen before include "Declaring or demanding emphatically" , "Demanding attention" , "Compelling attention" , "Constant" , "Unrelenting" .)