Old man on a climb, surmounting former peak (4)
I believe the answer is:
apex
'peak' is the definition.
(I know that peak can be written as apex)
'old man on a climb surmounting former' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'on a climb' shows that the letters should be reversed in order.
'surmounting' says to put letters next to each other (in a down clue, some letters go on top of others).
'former' becomes 'ex' (prefix meaning former or onetime).
'pa' reversed gives 'ap'.
'ap'+'ex'='APEX'
(Other definitions for apex that I've seen before include "Uppermost tip" , "Point or summit" , "Top, highest part" , "The highest point of something" , "Culmination" .)