Caught alien restraining knight and struck (9)
I believe the answer is:
entrapped
'caught' is the definition.
(entrapping is a kind of catching)
'alien restraining knight and struck' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'restraining' is an insertion indicator.
'knight' becomes 'n' (chess).
'and' says to put letters next to each other.
'struck' becomes 'rapped' (rapping is a kind of striking).
'et' going around 'n' is 'ent'.
'ent'+'rapped'='ENTRAPPED'
(Other definitions for entrapped that I've seen before include "unable to find a way out" , "Caught in a snare" , "Maybe framed" , "Snared" .)