See mother upset by the triplets
I believe the answer is:
threesome
'the triplets' is the definition.
(I've seen this before)
'see mother upset' is the wordplay.
'upset' indicates an anagram.
'see'+'mother'='seemother'
'seemother' with letters rearranged gives 'THREESOME'.
'by' acts as a link.
(Other definitions for threesome that I've seen before include "Trio" , "a crowd, perhaps" , "Chekhov's sisters" , "Small group of people" , "Triad" .)