Border guards love to get on (5)
I believe the answer is:
about
'on' is the definition.
(thesaurus)
'border guards love' is the wordplay.
'border' becomes 'abut' ('abut' can be a synonym of 'border').
'guards' indicates putting letters inside.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'abut' going around 'o' is 'ABOUT'.
'to get' acts as a link.
(Other definitions for about that I've seen before include "Circa" , "Concerning or in neighbourhood" , "Not exactly - concerning" , "Encompassing" , "Concerning; roughly" .)