Attempts to cross French sea, being back in boats (8)
I believe the answer is:
triremes
'boats' is the definition.
(I've seen this before)
'attempts to cross french sea being back' is the wordplay.
'attempts' becomes 'tries' ('try' can be a synonym of 'attempt').
'to cross' means one lot of letters goes inside another.
'french sea' becomes 'mer' ('sea' in French).
'being back' is a reversal indicator.
'mer' reversed gives 'rem'.
'tries' going around 'rem' is 'TRIREMES'.
'in' acts as a link.
(Other definitions for triremes that I've seen before include "They had three banks" , "boats" , "Galleys" .)