Like a fruitcake, only more so (7)
I believe the answer is:
nuttier
'like a' is the definition.
I can't tell whether this defines the answer.
'fruitcake only more so' is the wordplay.
'fruitcake' becomes 'nut' (I've seen this before).
'only more so' becomes 'tier' (I can't explain this - if you can you should give a lot more credence to this answer).
'nut'+'tier'='NUTTIER'
Can you help me to learn more?
(Other definitions for nuttier that I've seen before include "Increasingly unbalanced" , "That's not so sensible" .)