Magic Middle-earth monster blocking sides of Shire track (7)
I believe the answer is:
sorcery
'magic' is the definition.
(I know that sorcery is a type of magic)
'middle-earth monster blocking sides of shire track' is the wordplay.
'middle earth monster' becomes 'orc' (I've seen this in another clue).
'blocking' indicates putting letters inside (block can mean to plug a gap in something).
'sides of' means to remove the middle letters (only the letters on the sides remain).
'track' becomes 'ry' (abbreviation for railway).
'shire' with its centre removed is 'se'.
'orc' going into 'se' is 'sorce'.
'sorce'+'ry'='SORCERY'
(Other definitions for sorcery that I've seen before include "Use of magic" , "Magic; witchcraft" , "Necromancy" , "occultism" , "Black magic" .)