Most squiffy? Advice: short nap (8)
I believe the answer is:
tipsiest
'most squiffy?' is the definition.
('tipsy' can be a synonym of 'squiffy')
'advice short nap' is the wordplay.
'advice' becomes 'tip' (a tip is a piece of advice).
'short' means to remove the last letter.
'nap' becomes 'siesta' (siesta is a kind of nap).
'siesta' with its final letter taken away is 'siest'.
'tip'+'siest'='TIPSIEST'
(Other definitions for tipsiest that I've seen before include "Most mildly drunk or fuddled" , "Most tiddley" , "Most affected by booze" , "Most tired and emotional" , "Most tiddly" .)