Placing a short sentry near Jack’s car (6)
I believe the answer is:
jaguar
'jack's car' is the definition.
'jaguar' can be an answer for 'car' (I have seen 'posh car' mean 'jaguar' so perhaps 'car' could also mean 'jaguar'). I am not sure about the 'jack's' bit.
'placing a short sentry near' is the wordplay.
'placing' becomes 'ja' (I can't explain this - if you can you should believe this answer much more).
'a short' means to remove the last letter.
'sentry' becomes 'guard' (I've seen this before).
'near' means one lot of letters go next to another.
'guard' with its final letter removed is 'guar'.
'ja'+'guar'='JAGUAR'
Can you help me to learn more?
(Other definitions for jaguar that I've seen before include "S.Amer. wild cat" , "Wild cat from S America, or fast car" , "Large jungle cat" , "Spotted feline and prestigious car" , "Powerful cat" .)