Sign of an increase (4)
I believe the answer is:
plus
I believe this clue is a double definition.
'sign' is the first definition.
(plus sign is a kind of sign)
'an increase' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for plus that I've seen before include "Positive thing" , "Add" , "Positive sign" , "Added extra" , "As well as" .)