Suppose, taking helium in, to go up (8)
I believe the answer is:
theorise
'suppose' is the definition.
(synonyms)
'helium in to go up' is the wordplay.
'helium' becomes 'he' (He is the chemical symbol for helium).
'in' means one lot of letters goes inside another.
'go up' becomes 'rise' (rising is a kind of going up).
'he' going within 'to' is 'theo'.
'theo'+'rise'='THEORISE'
'taking' is the link.
(Other definitions for theorise that I've seen before include "Conjecture, hypothesise" , "Form view" , "Express ideas" , "Think up possible explanations" , "Suppose" .)