Afraid, when climbing, to pass over most of fire (8)
I believe the answer is:
timorous
'afraid' is the definition.
(thesaurus)
'when climbing to pass over most of fire' is the wordplay.
'when climbing' shows that the letters should be reversed in order (in a down clue, letters go up).
'to' becomes 'for'.
'pass over' becomes 'omit' (I've seen this in another clue).
'most' means to remove the first letter.
'of fire' becomes 'us' (I am not sure about this - if you are sure you should believe this answer much more).
'for'+'omit'='foromit'
'foromit' with its first letter taken away is 'oromit'.
'oromit' written backwards gives 'timoro'.
'timoro'+'us'='TIMOROUS'
Can you help me to learn more?
(Other definitions for timorous that I've seen before include "Shy, nervous" , "Nervous, fearful" , "Shrinking by nature" , "Shy, mouselike" , "Shy; retiring" .)