Turned when trapped (8)
I believe the answer is:
cornered
I believe this is a double definition.
'turned' is the first definition.
(cornering is a kind of turning)
'trapped' is the second definition.
(synonyms)
'when' acts as a link.
(Other definitions for cornered that I've seen before include "went round the bend" , "Caught in position with no escape, like rat" , "Monopolised the market" , "Having no way out" , "Trapped; rounded a bend" .)