Reduce size first to extract gunge (5)
I believe the answer is:
slime
'extract gunge' is the definition.
'slime' can be an answer for 'gunge' (I've seen this before). I'm not certain of the 'extract' bit.
'reduce size first' is the wordplay.
'reduce' becomes 'slim' (synonyms).
'size first' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'slim'+'e'='SLIME'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for slime that I've seen before include "could make things sticky" , "Gunge" , "Moist slippery stuff" , "Goo" , "Nasty slippery stuff" .)