Duke is grabbed by testicles; they are barely visible (7)

I believe the answer is:
nudists
'they are barely visible' is the definition.
The definition suggests an adverb but the answer is not. However, I know that 'barely' relates to this answer.
'duke is grabbed by testicles' is the wordplay.
'duke' becomes 'd' (abbreviation).
'grabbed by' is an insertion indicator (inserted letters are grabbed inside).
'testicles' becomes 'nuts' ('nut' can be a synonym of 'testicle').
'd'+'is'='dis'
'dis' put within 'nuts' is 'NUDISTS'.
Can you help me to learn more?
(Other definitions for nudists that I've seen before include "They're exposed" , "they reveal all" , "they're not in gear" , "Strippers?" , "Things abandoned by those" .)
