Smaller boy absent from lessons (4)
I believe the answer is:
less
'smaller' is the definition.
(thesaurus)
'boy absent from lessons' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy').
'absent from' suggests deleting specific letters.
'lessons' with 'son' taken out is 'LESS'.
(Other definitions for less that I've seen before include "Smaller in amout or degree" , "Fewer in number" , "In smaller quantity" , "minor" , "Fewer; minus" .)