Climbing skilfully, mostly, by one small Scottish island (5)
I believe the answer is:
lewis
'scottish island' is the definition.
(I've seen this before)
'climbing skilfully mostly by one small' is the wordplay.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'skilfully' becomes 'well' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'by one small' becomes 'is' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'well' with its final letter taken off is 'wel'.
'wel' in reverse letter order is 'lew'.
'lew'+'is'='LEWIS'
Can you help me to learn more?
(Other definitions for lewis that I've seen before include "Cecil Day ---, Poet Laureate" , "Scottish island" , "gunman?" , "- - Carroll, author" , "Other half of Harris (Hebrides)" .)