Some kitten seriously nervous (5)
I believe the answer is:
tense
'nervous' is the definition.
(similar in meaning)
'some kitten seriously' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'TENSE' is hidden within 'kitten seriously'.
(Other definitions for tense that I've seen before include "Taut - timely verbal indication" , "Charged" , "Stretched tight or rigid" , "Taut, strained" , "under stress?" .)