Tiny setter's amphibian is caught (6)
I believe the answer is:
minute
'tiny' is the definition.
(minute can mean small)
'setter's amphibian is caught' is the wordplay.
'setter's' becomes 'mine' (what belongs to the crossword setter).
'amphibian' becomes 'ut' (I am not sure about this - if you are sure you should believe this answer much more).
'is caught' means one lot of letters goes inside another.
'mine' placed around 'ut' is 'MINUTE'.
Can you help me to learn more?
(Other definitions for minute that I've seen before include "Really very small, like a period of time" , "Very small, like unit of time" , "Immeasurably small" , "Small, tiny" , "Tiny; period of time" .)