Daughters initially on ship wear formal clothes (5)
I believe the answer is:
dress
'wear formal clothes' is the definition.
(eg dressing for dinner)
'daughters initially on ship' is the wordplay.
'initially' says to take the initial letters.
'on' becomes 're' ('on' can be mean 'with reference to').
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
The first letter of 'daughters' is 'd'.
'd'+'re'+'ss'='DRESS'
(Other definitions for dress that I've seen before include "Garb" , "Costume" , "clean > bandage" , "female garment" , "Do some cover work in theatre" .)