Sting and Summers essentially invested in small paintings (5)
I believe the answer is:
smart
'sting' is the definition.
(I've seen this before)
'summers essentially invested in small paintings' is the wordplay.
'essentially' says to take the centre.
'invested in' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'paintings' becomes 'art' (I've seen this before).
The centre of 'summers' is 'm'.
's'+'art'='sart'
'm' put within 'sart' is 'SMART'.
'and' is the link.
(Other definitions for smart that I've seen before include "Spruce; sting" , "Fine" , "astute" , "it hurts!" , "Neat - clever" .)