Fit out start of every joke (5)
I believe the answer is:
equip
'fit out' is the definition.
(I know that fit out can be written as equip)
'start of every joke' is the wordplay.
'start of' indicates taking the first letters.
'joke' becomes 'quip' (quipping is a kind of joking).
The first letter of 'every' is 'e'.
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Accoutre" , "Furnish, fit out" , "Provide with suitable gear" , "Outfit" , "Furnish with supplies" .)