Aiming to follow foolish instruction in nursery (5-8)
I believe the answer is:
potty-training
'instruction in nursery' is the definition.
(children in nursery are potty-trained)
'aiming to follow foolish' is the wordplay.
'aiming' becomes 'training' ('train' can be a synonym of 'aim').
'to follow' means one lot of letters go next to another.
'foolish' becomes 'potty' (I've seen this before).
'training' put after 'potty' is 'POTTY-TRAINING'.
(Other definitions for potty-training that I've seen before include "Behavioural lessons for the very young" , "Post-nappy education?" .)