In class, natural to make slow progress (5)
I believe the answer is:
crawl
'slow progress' is the definition.
(I've seen this before)
'in class natural' is the wordplay.
'in' indicates putting letters inside.
'class' becomes 'cl' (I've seen this in other clues).
'natural' becomes 'raw' ('raw' can be a synonym of 'natural').
'cl' going around 'raw' is 'CRAWL'.
'to make' acts as a link.
(Other definitions for crawl that I've seen before include "Move slowly (as a baby does?)" , "Get along swimmingly" , "Progress, slow on land but fast in water" , "inch" , "Move as snakes do, or on hands and knees" .)