Playful law lord sick at heart (5)
I believe the answer is:
ludic
'playful' is the definition.
(I've seen this in another clue)
'law lord sick at heart' is the wordplay.
'law lord' becomes 'lud' (term used in law as in m'lud, my lord).
'at heart' indicates the central letters.
The middle of 'sick' is 'ic'.
'lud'+'ic'='LUDIC'
(Other definitions for ludic that I've seen before include "Playful; lucid (anag.)" , "Playing aimlessly" .)