Speculate that a man has to get up (8)
I believe the answer is:
theorise
'speculate' is the definition.
(I know that speculate can be written as theorise)
'a man has to get up' is the wordplay.
'a man' becomes 'theo' (I've seen this before).
'has' says to put letters next to each other.
'to get up' becomes 'rise' ('rise' can be a synonym of 'get up').
'theo'+'rise'='THEORISE'
'that' is the link.
(Other definitions for theorise that I've seen before include "Speculate by reasoning" , "Conjecture, hypothesise" , "Think up possible explanations" , "Express ideas" , "Form of hypothesis or conjecture" .)