Rocks when I change ends (3)
I believe the answer is:
ice
'rocks' is the definition.
(I've seen this before)
'i change ends' is the wordplay.
'ends' says to hollow out the word (remove centre letters).
'change' with its middle removed is 'ce'.
'i'+'ce'='ICE'
'when' is the link.
(Other definitions for ice that I've seen before include "Freeze" , "Being feeble, can't cut any" , "Skating surface" , "engine" , "Rocks on which drink is served?" .)