Understood mischievous child was untruthful (7)
I believe the answer is:
implied
'understood' is the definition.
(thesaurus)
'mischievous child was untruthful' is the wordplay.
'mischievous child' becomes 'imp' (I've seen this before).
'was untruthful' becomes 'lied' (I've seen this before).
'imp'+'lied'='IMPLIED'
(Other definitions for implied that I've seen before include "Insinuated, hinted" , "Suggested without stating directly" , "Understood" , "Unspoken" , "Suggested; not explicit" .)