Studied this writer on a certain day (8)
I believe the answer is:
measured
'studied' is the definition.
(thesaurus)
'writer on a certain day' is the wordplay.
'writer' becomes 'me' (the person who wrote the clues).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'certain' becomes 'sure' (synonyms).
'day' becomes 'd' (abbreviation for day).
'me'+'a'+'sure'+'d'='MEASURED'
'this' is the link.
(Other definitions for measured that I've seen before include "Weighed" , "Found the size or quantity of something - using tape or ruler" , "Made sure (anag.)" , "Metered" , "Calculated, quantified" .)