Pair sunbathe by sea, rolling over (6)
I believe the answer is:
tandem
'pair' is the definition.
(thesaurus)
'sunbathe by sea rolling over' is the wordplay.
'sunbathe' becomes 'tan' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'sea' becomes 'med' (abbreviation of Mediterranean).
'rolling over' shows that the letters should be reversed in order.
'med' written backwards gives 'dem'.
'tan'+'dem'='TANDEM'
(Other definitions for tandem that I've seen before include "Bike with two saddles" , "Bicycle for two (plus)" , "Bicycle built for two" , "Bicycle for two riders" , "Two-seat bicycle" .)