Impenetrable studies before end of course (5)
I believe the answer is:
dense
'impenetrable' is the definition.
('dense' can be similar in meaning to 'impenetrable')
'studies before end of course' is the wordplay.
'studies' becomes 'dens' (I've seen this before).
'before' says to put letters next to each other.
'end' indicates one should take the final letters.
The last letter of 'course' is 'e'.
'dens'+'e'='DENSE'
(Other definitions for dense that I've seen before include "Needs (anag) -- thick" , "Slow-witted" , "Closely compacted" , "Thick; stupid" , "Close knit" .)