A reason for being beached? (7)
I believe the answer is:
aground
'being beached?' is the definition.
('aground' can be similar in meaning to 'beached')
'a reason' is the wordplay.
'reason' becomes 'ground' (synonyms).
'a'+'ground'='AGROUND'
'for' acts as a link.
(Other definitions for aground that I've seen before include "(Ship) unable to float" , "High and dry" , "Unable to move" , "(Of a ship) stuck on the seabed" , "No longer floating" .)