Men climbing caught by enormous bird (3)
I believe the answer is:
roc
'enormous bird' is the definition.
(I've seen this before)
'men climbing caught by' is the wordplay.
'men' becomes 'or' (I've seen this in other clues**).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'caught' becomes 'c' (cricket abbreviation).
'by' is a charade indicator (letters next to each other).
'or' in reverse letter order is 'ro'.
'ro'+'c'='ROC'
(Other definitions for roc that I've seen before include "Sinbad's huge bird" , "Enormous legendary bird" , "Big bird" , "Legendary thing" , "Gigantic bird of eastern legend - cor!" .)