Superman will occasionally sit in an ungainly manner (6)
I believe the answer is:
sprawl
'sit in an ungainly manner' is the definition.
(I've seen this before)
'superman will occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'superman'+'will'='supermanwill'
The alternating letters of 'supermanwill' are 'SPRAWL'.
(Other definitions for sprawl that I've seen before include "Sit lazily, with limbs spread wide" , "Lounge" , "Spread out untidily" , "What stragglers do" , "Spread out in ungainly way" .)