Riddle of diamonds finally found on coach (8)
I believe the answer is:
strainer
'riddle' is the definition.
(thesaurus)
'diamonds finally found on coach' is the wordplay.
'finally' says to take the final letters.
'found on' says to put letters next to each other.
'coach' becomes 'trainer' (I've seen this before).
The last letter of 'diamonds' is 's'.
's'+'trainer'='STRAINER'
'of' acts as a link.
(Other definitions for strainer that I've seen before include "Tea leaf remover" , "one full of holes" , "Sieve, colander, etc" , "who tries really hard?" , "one that stretches?" .)