Stuck aluminium in a cast (2,1,4)
I believe the answer is:
at a loss
'stuck' is the definition.
(thesaurus)
'aluminium in a cast' is the wordplay.
'aluminium' becomes 'al' ('Al' can be a synonym of 'aluminium').
'in' is an insertion indicator.
'cast' becomes 'toss' (I've seen this before**).
'a'+'toss'='atoss'
'al' placed within 'atoss' is 'AT A LOSS'.
(Other definitions for at a loss that I've seen before include "how incompetent companies can operate" , "how unsuccessful business trades?" , "Puzzled - below cost" , "How unprofitable business is run" , "how a bankrupt business was run" .)