Very nearly locating obscure rocks (5,3,2,5)
I believe the answer is:
close but no cigar
'very nearly' is the definition.
(I've seen this before)
'locating obscure rocks' is the wordplay.
'rocks' indicates anagramming the letters.
'locating'+'obscure'='locatingobscure'
'locatingobscure' is an anagram of 'CLOSE BUT NO CIGAR'.
(Other definitions for close but no cigar that I've seen before include "not good enough for Havana?" , "Very nearly" , "It's not quite good enough" .)