I am unhappy without 50 to inspire (5)
I believe the answer is:
imbue
'to inspire' is the definition.
(I've seen this before)
'i am unhappy without 50' is the wordplay.
'i am' becomes 'I'm' (contraction).
'unhappy' becomes 'blue' (blue can mean sad or miserable).
'without' is a deletion indicator.
'50' becomes 'l' (Roman numeral).
'blue' with 'l' removed is 'bue'.
'im'+'bue'='IMBUE'
(Other definitions for imbue that I've seen before include "Endow" , "dye" , "Spread or diffuse through; inspire" , "Suffuse - soak" , "Charge" .)