This writer is one day behind rear of coconut shy (5)
I believe the answer is:
timid
'shy' is the definition.
(synonyms)
'this writer is one day behind rear of coconut' is the wordplay.
'this writer is' becomes 'I'm' (I am - referring to the crossword setter).
'one' becomes 'i' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'behind' means one lot of letters go next to another.
'rear of' suggests the final letters.
The last letter of 'coconut' is 't'.
'im'+'i'+'d'='imid'
'imid' after 't' is 'TIMID'.
(Other definitions for timid that I've seen before include "Lacking self-confidence" , "Nervous and easily alarmed" , "Shrinking" , "Easily frightened" , "Not assertive, retiring" .)