Ordinary crustacean up rocky end of pool feeds (13)
I believe the answer is:
unspectacular
'ordinary' is the definition.
(I've seen this before)
'crustacean up rocky end of pool feeds' is the wordplay.
'rocky' indicates an anagram.
'end of' says to take the final letters.
'feeds' means one lot of letters goes inside another (one word is fed by another set of letters).
The final letter of 'pool' is 'l'.
'crustacean'+'up'='crustaceanup'
'crustaceanup' with letters rearranged gives 'unspectacuar'.
'unspectacuar' going around 'l' is 'UNSPECTACULAR'.
(Other definitions for unspectacular that I've seen before include "Ordinary" , "Not dramatic" , "Unremarkable" .)