Studies the East - it's thickly populated (5)
I believe the answer is:
dense
'thickly populated' is the definition.
(I've seen this in another clue)
'studies the east' is the wordplay.
'studies' becomes 'dens' (I've seen this before).
'the east' becomes 'E' (synonyms).
'dens'+'e'='DENSE'
'it's' acts as a link.
(Other definitions for dense that I've seen before include "Closely crowded together" , "Obtuse - impenetrable" , "Close-packed" , "Closely spaced or extremely stupid" , "Close knit" .)