A sufficient reason for being unable to float (7)
I believe the answer is:
aground
'being unable to float' is the definition.
I can't tell whether this defines the answer.
'a sufficient reason' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'reason' could be 'ground' (synonyms) and 'ground' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for aground that I've seen before include "out of water" , "(Of ship) touching the bottom" , "Unable to move" , "(Of ship) not floating" , "on the bed" .)