Partners found parking charges steep with Sierra (7)
I believe the answer is:
spouses
'partners' is the definition.
(I know this)
'parking charges steep with sierra' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'charges' indicates putting letters inside (charge can mean to charge into something).
'steep' becomes 'souse' (both can mean to submerge in liquid).
'with' says to put letters next to each other.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'p' placed within 'souse' is 'spouse'.
'spouse'+'s'='SPOUSES'
'found' acts as a link.
(Other definitions for spouses that I've seen before include "Wives or husbands" , "Partners in marriage" .)