Copy but make it smaller, for fitting inside (9)
I believe the answer is:
reproduce
'copy' is the definition.
(I know that copy is a more specific form of the action double)
'for fitting inside' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'fitting' becomes 'reduce' (I can't explain this - if you can you should believe this answer much more).
'inside' means one lot of letters goes inside another.
'pro' placed within 'reduce' is 'REPRODUCE'.
'but make it smaller' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for reproduce that I've seen before include "achieve propagation" , "[DOUBLE]" , "Ape" , "Procreate or make a copy" , "To propagate" .)