Third eye oddly substantial (4)
I believe the answer is:
tidy
'substantial' is the definition.
(as in a tidy sum of money)
'third eye oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'third'+'eye'='thirdeye'
The alternate letters of 'thirdeye' are 'TIDY'.
(Other definitions for tidy that I've seen before include "Spruce up" , "Spick-and-span" , "Set in order" , "Uncluttered" , "Clear up" .)