I will need energy to travel (3)
I believe the answer is:
ego
'i' is the definition.
(thesaurus)
'energy to travel' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'to travel' becomes 'go' ('go' can be a synonym of 'travel').
'e'+'go'='EGO'
'will need' is the link.
(Other definitions for ego that I've seen before include "Selfishness" , "In Virgil I" , "Self-confidence" , "Part of the mind" , "One's image of oneself" .)