Crave to become a speleologist (5)
I believe the answer is:
caver
'a speleologist' is the definition.
(I've seen this before)
'crave to become' is the wordplay.
'to become' is an anagram indicator.
'crave' with letters rearranged gives 'CAVER'.
(Other definitions for caver that I've seen before include "Speleologist" , "Potholer" , "Underground explorer" .)