Fifty-four ways to buck up (5)
I believe the answer is:
liven
'buck up' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'fifty-four ways' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'four' becomes 'IV' (Roman numerals).
'ways' becomes 'en' (I can't explain this - if you can you should believe this answer much more).
'l'+'iv'+'en'='LIVEN'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for liven that I've seen before include "Invigorate" , "Animate, make energetic" , "Cheer up" , "Cause excitement" , "Make active" .)