Mysterious gain with climate fluctuating (11)
I believe the answer is:
enigmatical
'mysterious' is the definition.
(I've seen this before)
'gain with climate fluctuating' is the wordplay.
'with' is a charade indicator (letters next to each other).
'fluctuating' indicates anagramming the letters.
'gain' put after 'climate' is 'climategain'.
'climategain' with letters rearranged gives 'ENIGMATICAL'.
(Other definitions for enigmatical that I've seen before include "about puzzles" , "Mysterious" , "Puzzling" .)