Joanna's leg enthralling a head of orthopaedics (5)
I believe the answer is:
piano
'joanna's' is the definition.
(cockney rhyming slang for piano)
'leg enthralling a head of orthopaedics' is the wordplay.
'leg' becomes 'pin' (pin is a kind of leg).
'enthralling' indicates putting letters inside.
'head of' suggests taking the first letters.
The first letter of 'orthopaedics' is 'o'.
'pin' going around 'a' is 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "In music, soflty" , "Franz Liszt's instrument" , "entertain with this softly" , "Upright type?" , "Clavier" .)