Make slow progress, being lazy at first in school (4)
I believe the answer is:
'make slow progress' is the definition.
(I've seen this before)
'lazy at first in school' is the wordplay.
'at first' indicates taking the first letters.
'in' indicates putting letters inside.
'school' becomes 'pod' (term for a group of marine mammals).
The first letter of 'lazy' is 'l'.
'l' placed inside 'pod' is 'PLOD'.
'being' is the link.
(Other definitions for plod that I've seen before include "Heavy walk; PC (slang)" , "Walk with effort" , "Shamus, say" , "coppers?" , "Tramp along" .)