What pupils should pay? A strain, by the sound of it (9)
I believe the answer is:
attention
'what pupils should pay?' is the definition.
(pupils should pay attention to their teacher)
'a strain by the sound of it' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'of' could be 'o' and 'o' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This explanation may well be incorrect...
(Other definitions for attention that I've seen before include "Concentration, heed" , "Pay it and concentrate" , "Special care, soldier's position" , "Application, concentration" , "Applicaton, concentration" .)