Sensible to follow sourpuss, but not straightforwardly (8)
I believe the answer is:
crabwise
'not straightforwardly' is the definition.
(to move crabwise is to move sideways)
'sensible to follow sourpuss' is the wordplay.
'sensible' becomes 'wise'.
'to follow' means one lot of letters go next to another.
'sourpuss' becomes 'crab' (both can mean a grumpy person).
'wise' after 'crab' is 'CRABWISE'.
'but' is the link.
(Other definitions for crabwise that I've seen before include "(Of movement) sideways" , "not in a straightforward way" , "to one side?" .)