Concentrated on treating tennis elbow initially (7)
I believe the answer is:
intense
'concentrated' is the definition.
(similar in meaning)
'treating tennis elbow initially' is the wordplay.
'treating' indicates anagramming the letters.
'initially' indicates taking the first letters.
The initial letter of 'elbow' is 'e'.
'tennis' anagrammed gives 'intens'.
'intens'+'e'='INTENSE'
'on' is the link.
(Other definitions for intense that I've seen before include "Deeply emotional" , "burning" , "'Acute, extreme (7)'" , "Fervent" , "Powerful" .)