Shepherd and sheep going over sedimentary rock, mostly (7)
I believe the answer is:
marshal
'shepherd' is the definition.
(both can mean to gather)
'sheep going over sedimentary rock mostly' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'going over' is a reversal indicator.
'sedimentary rock' becomes 'shale' (shale is a kind of sedimentary rock).
'mostly' means to remove the last letter.
'ram' backwards is 'mar'.
'shale' with its last letter taken away is 'shal'.
'mar'+'shal'='MARSHAL'
'and' is the link.
(Other definitions for marshal that I've seen before include "guide" , "Top officer" , "Law officer in the old Wild West" , "High officer" , "High-ranking officer" .)