One stands in pass, climbing with hesitation (5)
I believe the answer is:
locum
'one stands in' is the definition.
(I've seen this before)
'pass climbing with hesitation' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'with' means one lot of letters go next to another.
'hesitation' becomes 'um' (um is said in hesitation).
'col' reversed gives 'loc'.
'loc'+'um'='LOCUM'
(Other definitions for locum that I've seen before include "Temporary deputy for doctor" , "temporary occupant" , "One stands in" , "Stand-in GP" , "Professional who substitutes temporarily for another" .)