Time in playing tennis is demanding (9)
I believe the answer is:
insistent
'demanding' is the definition.
(I've seen this in another clue)
'time in playing tennis is' is the wordplay.
'time' becomes 't' (abbreviation).
'in' means one lot of letters goes inside another.
'playing' is an anagram indicator.
'tennis'+'is'='tennisis'
'tennisis' is an anagram of 'insisent'.
't' put within 'insisent' is 'INSISTENT'.
(Other definitions for insistent that I've seen before include "Determined" , "Demanding attention" , "Unwilling to take "no" for an answer" , "Not giving up" , "Declaring or demanding emphatically" .)