Whole exploit receiving tense thanks in high-level meeting (6,4)

'high-level meeting' is the definition.

'whole exploit receiving tense thanks' is the wordplay.
'whole' becomes 'sum' (sum is a kind of whole).
'exploit' becomes 'milk' (to milk something is to exploit its potential).
'receiving' indicates putting letters inside.
'tense' becomes 't' (abbreviation used in many dictionaries).
'thanks' becomes 'ta' (UK informal version of 'thank you').
't'+'ta'='tta'
'milk' going around 'tta' is 'mittalk'.
'sum'+'mittalk'='SUMMIT TALK'

'in' acts as a link.