A large limb aquatic mammal raised briskly (10)
I believe the answer is:
allegretto
'briskly' is the definition.
(musical instruction)
'a large limb aquatic mammal raised' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'limb' becomes 'leg' (leg is a kind of limb).
'aquatic mammal' becomes 'otter' (otters live by water).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'otter' back-to-front is 'retto'.
'a'+'l'+'leg'+'retto'='ALLEGRETTO'
(Other definitions for allegretto that I've seen before include "somewhat quick" , "quite quickly" , "fairly speedily" , "with reasonable rapidity" , "quite fast" .)