Clothe Nick in nurse's costume at last (6)
I believe the answer is:
enrobe
'clothe' is the definition.
(I've seen this before)
'nick in nurse's costume at last' is the wordplay.
'nick' becomes 'rob' (both can mean to steal).
'in' means one lot of letters goes inside another.
'nurse' becomes 'EN' (abbreviation for state-enrolled nurse).
'at last' indicates one should take the final letters.
The final letter of 'costume' is 'e'.
'en'+'e'='ene'
'rob' placed within 'ene' is 'ENROBE'.
(Other definitions for enrobe that I've seen before include "Fancy" , "Dress, clothe, invest" , "put on formally ..." , "Dress, put on vestments" , "Clad ceremonially" .)