Tart up, on prodding feather once? (5)
I believe the answer is:
preen
'tart up' is the definition.
(both can mean to make oneself look more attractive)
'on prodding feather once?' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'prodding' means one lot of letters goes inside another (prod can mean to poke a hole into something).
'feather once?' becomes 'pen' (feathers were once used as pens or quills).
're' inserted into 'pen' is 'PREEN'.
(Other definitions for preen that I've seen before include "Array (oneself) carefully" , "Self-congratulate" , "Dress or smooth, feathers say" , "Groom elaborately" , "Titivate oneself" .)