Using its tail, a monkey could open a door! (3)
I believe the answer is:
key
'open a door' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'using its tail a monkey' is the wordplay.
'KEY' is hidden in amongst the letters.
But, I'm uncertain how the hidden word is indicated.
'could' is the link.
Can you help me to learn more?
(Other definitions for key that I've seen before include "Device for lock" , "Clue" , "Lock-opener" , "Eg, Ctrl, Alt or Esc" , "locker" .)