A trek's leader packing cross and lucky charm (6)
I believe the answer is:
amulet
'lucky charm' is the definition.
(I've seen this before)
'a trek's leader packing cross' is the wordplay.
'leader' says to take the initial letters.
'packing' is an insertion indicator.
'cross' becomes 'mule' (I've seen this before**).
The first letter of 'treks' is 't'.
'a'+'t'='at'
'at' placed around 'mule' is 'AMULET'.
'and' acts as a link.
(Other definitions for amulet that I've seen before include "Protective trinket" , "Ornament worn as a charm against evil" , "Protective charm" , "Charm worn against bad luck" , "Small item of jewellery" .)