A singular reason for being beached (7)
I believe the answer is:
aground
'being beached' is the definition.
(similar in meaning)
'a singular reason' is the wordplay.
'a singular' becomes 'a' (both can mean one).
'reason' becomes 'ground' (synonyms).
'a'+'ground'='AGROUND'
'for' acts as a link.
(Other definitions for aground that I've seen before include "(Of ship) not floating" , "Beached" , "No longer floating" , "out of water" , "having hit rock bottom, perhaps" .)