Playing tennis I have concentrated (9)
I believe the answer is:
intensive
'concentrated' is the definition.
('intensive' can be a synonym of 'concentrated')
'playing tennis i have' is the wordplay.
'playing' indicates an anagram.
'i have' becomes 'ive'.
'tennis' anagrammed gives 'intens'.
'intens'+'ive'='INTENSIVE'
(Other definitions for intensive that I've seen before include "Concentrated" , "Very vigorous or thorough; concentrating" , "Demanding" .)