Knucklehead initially crawls into sand ridge (5)
I believe the answer is:
dunce
'knucklehead' is the definition.
(synonyms)
'initially crawls into sand ridge' is the wordplay.
'initially' indicates taking the first letters.
'into' means one lot of letters goes inside another.
'sand ridge' becomes 'dune' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'crawls' is 'c'.
'c' inserted into 'dune' is 'DUNCE'.
Can you help me to learn more?
(Other definitions for dunce that I've seen before include "Blockhead" , "Slow-learner" , "Duffer" , "School fool" , "Poor scholar, slow at learning" .)