Travel essential for Dad's hobby (8)
I believe the answer is:
passport
'travel essential' is the definition.
'dad's hobby' is the wordplay.
'dad's' becomes 'pas' ('pa' can be a synonym of 'dad').
'hobby' becomes 'sport'.
'pas'+'sport'='PASSPORT'
'for' is the link.
(Other definitions for passport that I've seen before include "International identification document" , "Document allowing travel abroad" , "Travel authorisation" , "Official document required for foreign travel" , "you might need Visa with this" .)