Also used by Highlanders (3)
I believe the answer is:
and
'also' is the definition.
(both can mean in addition)
'used by highlanders' is the wordplay.
'used by' says the answer is hidden in the clue.
'AND' can be found hidden inside 'highlanders'.
(Other definitions for and that I've seen before include "Plus" , "A very common conjunction" , "Together with" , "It's used to connect words, clauses or sentences" , "Joining word" .)