A Greek mountain could be so lumpy! (7)
I believe the answer is:
olympus
'a greek mountain' is the definition.
(I've seen this before)
'could be so lumpy' is the wordplay.
'could be' indicates an anagram.
'so'+'lumpy'='solumpy'
'solumpy' with letters rearranged gives 'OLYMPUS'.
(Other definitions for olympus that I've seen before include "Mountain - dwelling place of the gods" , "In Greek myth, Mount ....... was home of the Gods" , "Greece's highest mountain" , "Mountain home of the Greek gods" , "Mount of the ancient Greek gods" .)