Against one mischievous child nothing has a tremulous effect (7)
I believe the answer is:
vibrato
'a tremulous effect' is the definition.
'vibrato' can be an answer for 'effect' (I have seen 'Oscillatory effect' mean 'vibrato' so perhaps 'effect' could also mean 'vibrato'). I'm not sure about the 'a tremulous' bit.
'against one mischievous child nothing has' is the wordplay.
'against' becomes 'v' (versus).
'one' becomes 'i' (Roman numeral).
'mischievous child' becomes 'brat' (I've seen this in another clue).
'nothing has' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'v'+'i'+'brat'+'o'='VIBRATO'
Can you help me to learn more?
(Other definitions for vibrato that I've seen before include "Singing effect" , "Rapid pitch variation" , "Pulsating effect in instrumental or vocal tone" , "Oscillatory effect" , "Rapid alternation in pitch" .)