Old man crosses line climbing mountain (3)
I believe the answer is:
alp
'mountain' is the definition.
(alp is a kind of mountain)
'old man crosses line climbing' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'crosses' is an insertion indicator.
'line' becomes 'l'.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'pa' going around 'l' is 'pla'.
'pla' written backwards gives 'ALP'.
(Other definitions for alp that I've seen before include "Swiss mountain" , "Mont Blanc, for example" , "high point" , "One of a European mountain range" , "Mountain pasture" .)