Grant, going by unclothed, receiving ultimate in catcalling (6)
I believe the answer is:
assign
'grant' is the definition.
(I've seen this before)
'going by unclothed receiving ultimate in catcalling' is the wordplay.
'going by' becomes 'passing' ('pass' can be a synonym of 'go by').
'unclothed' indicates the central letters.
'receiving' indicates putting letters inside.
'ultimate in' suggests the final letters.
The central letters of 'passing' are 'assin'.
The final letter of 'catcalling' is 'g'.
'assin' going around 'g' is 'ASSIGN'.
(Other definitions for assign that I've seen before include "Appoint to a duty" , "Allocate (duties to someone)" , "(Of a job) allocate to someone" , "Transfer or appoint to" , "Transfer (legal rights)" .)