Dad boards ship for restful resorts (4)
I believe the answer is:
spas
'restful resorts' is the definition.
(I've seen this in another clue)
'dad boards ship' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'boards' indicates putting letters inside.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'pa' going into 'ss' is 'SPAS'.
'for' acts as a link.
(Other definitions for spas that I've seen before include "Mineral springs" , "Bath and suchlike" , "Health resorts near springs" .)