Add to henna spread on skin of cattle (7)
I believe the answer is:
enhance
'add to' is the definition.
(thesaurus)
'henna spread on skin of cattle' is the wordplay.
'spread' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'skin of' means to remove the middle letters.
'cattle' with its middle removed is 'ce'.
'henna' with letters rearranged gives 'enhan'.
'enhan'+'ce'='ENHANCE'
(Other definitions for enhance that I've seen before include "Improve, add to attractiveness of" , "'Improve, augment (7)'" , "Boost" , "Make better or more attractive" , "Add function, appeal, to" .)