A child to tease without rest (3)
I believe the answer is:
tot
'a child' is the definition.
(tot is a kind of child)
'to tease without rest' is the wordplay.
'without' indicates named letters should be taken away.
'rest' becomes 'ease' ('ease' can be a synonym of 'rest').
'tease' with 'ease' taken away is 't'.
'to'+'t'='TOT'
(Other definitions for tot that I've seen before include "Child's" , "Drop" , "Small quantity of alcoholic drink" , "Add up like infant" , "Arithmetic for infant" .)