Tiny cat, one trapped by current (6)
I believe the answer is:
atomic
'tiny' is the definition.
'cat one trapped by current' is the wordplay.
'cat' becomes 'tom' (tom is a kind of cat).
'one' becomes 'i' (Roman numeral).
'trapped by' means one lot of letters goes inside another.
'current' becomes 'ac' (alternating current).
'tom'+'i'='tomi'
'tomi' inserted into 'ac' is 'ATOMIC'.
(Other definitions for atomic that I've seen before include "Using nuclear energy" , "Sort of bomb, number, or power" , "Describing tiny particles" , "Realting to nuclear energy" , "Referring to the smallest component" .)