Climbing part of Munro, dalesman, trim (5)
I believe the answer is:
adorn
'trim' is the definition.
(I've seen this in another clue)
'climbing part of munro dalesman' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'part of' says the answer is hidden in the clue.
'nroda' is hidden within 'munro dalesman'.
'nroda' in reverse letter order is 'ADORN'.
(Other definitions for adorn that I've seen before include "dress" , "Improve appearance" , "Deck" , "Beautify - decorate - enhance" , "Festoon" .)