Cat starts to inhale catnip after a minute (6)
I believe the answer is:
atomic
'minute' is the definition.
(synonyms)
'cat starts to inhale catnip after a' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'starts to' suggests taking the first letters.
'after' means one lot of letters go next to another.
The initial letters of 'inhale catnip' is 'ic'.
'tom'+'ic'='tomic'
'tomic' put after 'a' is 'ATOMIC'.
(Other definitions for atomic that I've seen before include "Sort of bomb, number, or power" , "Like some clocks" , "Energy released in nuclear fission or fusion" , "Realting to nuclear energy" , "Type of energy" .)