Healthy muscle protects this gland (6)
I believe the answer is:
thymus
'this gland' is the definition.
(thymus is a kind of gland)
'healthy muscle protects' is the wordplay.
'protects' indicates the answer is hidden within the clue (hidden letters are protected by the clue).
'THYMUS' is hidden within 'healthy muscle'.
(Other definitions for thymus that I've seen before include "Organ" , "A gland in neck" .)