Tough lioness ultimately keeping old squirrels away (6)
I believe the answer is:
hoards
'squirrels away' is the definition.
(I've seen this in another clue)
'tough lioness ultimately keeping old' is the wordplay.
'tough' becomes 'hard' (similar in meaning).
'ultimately' indicates one should take the final letters.
'keeping' is an insertion indicator (some letters kept inside).
'old' becomes 'o' (abbreviation).
The last letter of 'lioness' is 's'.
'hard'+'s'='hards'
'hards' placed around 'o' is 'HOARDS'.
(Other definitions for hoards that I've seen before include "Hidden accumulations" , "Amasses" , "Squirrels away" , "Stockpiles" , "Stores up treasure" .)