Monkey occupying what must be reserved for clinger (6)
I believe the answer is:
limpet
'reserved for clinger' is the definition.
The definition suggests a singular noun which matches the answer.
'monkey occupying what' is the wordplay.
'monkey' becomes 'imp' (synonyms).
'occupying' means one lot of letters goes inside another.
'what' becomes 'let' (I can't explain this - if you can you should believe this answer much more).
'imp' inserted into 'let' is 'LIMPET'.
'must be' is the link.
Can you help me to learn more?
(Other definitions for limpet that I've seen before include "Clinging gastropod" , "Rockpool clinger?" , "Clingy person" , "he's not easily got rid of" , "It clings to rock surfaces" .)