Piano stool leg occasionally missing pad under foot (6)
I believe the answer is:
insole
'pad under foot' is the definition.
(foot support inside a shoe)
'piano stool leg occasionally missing' is the wordplay.
'occasionally missing' indicates alternate letters (letters removed at regular intervals or occasions).
'piano'+'stool'+'leg'='pianostoolleg'
The alternate letters of 'pianostoolleg' are 'INSOLE'.
(Other definitions for insole that I've seen before include "support for rambler, perhaps" , "Lesion in part of foot" , "found in boot" , "Padding in a shoe" , "Part of Oxford perhaps" .)