Sailors discovered hip bones (5)
I believe the answer is:
tarsi
'bones' is the definition.
(bones of the foot)
'sailors discovered hip' is the wordplay.
'sailors' becomes 'tars' (tar is a kind of sailor).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
The middle of 'hip' is 'i'.
'tars'+'i'='TARSI'
(Other definitions for tarsi that I've seen before include "Bones of the ankles and heels" , "Foot bones" , "Ankle bones" , "Parts of skeleton" , "collections of bones?" .)