Moor that is below top of tor (3)
I believe the answer is:
tie
'moor' is the definition.
(thesaurus)
'that is below top of tor' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'below' says to put letters next to each other.
'top of' says to take the initial letters.
The initial letter of 'tor' is 't'.
'ie' put after 't' is 'TIE'.
(Other definitions for tie that I've seen before include "Fasten, link to" , "Neck-cloth" , "restraint" , "Secure (with string)" , "Affiliation" .)