500 Royal Engineers put on ship's clothing (5)
I believe the answer is:
dress
'clothing' is the definition.
(dress is a kind of clothing)
'500 royal engineers put on ship's' is the wordplay.
'500' becomes 'd' ('d' can be a synonym of '500').
'royal engineers' becomes 're'.
'put on' means one lot of letters go next to another.
'ship' becomes 'ss'.
'd'+'re'+'ss'='DRESS'
(Other definitions for dress that I've seen before include "what she wears?" , "items to wear" , "Costume" , "deck" , "Robe" .)