Sailor in small tub for rest day (7)
I believe the answer is:
sabbath
'for rest day' is the definition.
(Sabbath is an example)
'sailor in small tub' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tub' becomes 'bath' (synonyms).
's'+'bath'='sbath'
'ab' placed inside 'sbath' is 'SABBATH'.
(Other definitions for sabbath that I've seen before include "Jewish rest-day" , "Day of rest and worship" , "Traditional holy day" , "Hebrew word for day of rest" , "Sacred day of the week" .)