Dad struggling with knee, which is massaged (7)
I believe the answer is:
kneaded
'is massaged' is the definition.
(I've seen this in another clue)
'dad struggling with knee' is the wordplay.
'struggling with' indicates an anagram (the letters struggle or wriggle into a new form).
'dad' anagrammed with 'knee' makes 'KNEADED'.
'which' acts as a link.
(Other definitions for kneaded that I've seen before include "Having made bloomer, perhaps" , "Worked into dough with the hands" .)