Held on to key organ (5)
I believe the answer is:
clung
'held on' is the definition.
(I've seen this in another clue)
'key organ' is the wordplay.
'key' becomes 'c' (musical key).
'organ' becomes 'lung' (lung is a kind of organ).
'c'+'lung'='CLUNG'
'to' acts as a link.
(Other definitions for clung that I've seen before include "Held on tight" , "Held on tightly" , "Held on desperately" , "Was emotionally close" .)