For work meetings at home, at first get into underclothes (9)

'for work meetings' is the definition.
(a briefing is a meeting providing information)

'home at first get into underclothes' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'at' means one lot of letters go next to another.
'first get' becomes 'g' (1st letter of 'get').
'into' means one lot of letters goes inside another.
'underclothes' becomes 'briefs' (type of underwear).
'in'+'g'='ing'
'ing' put inside 'briefs' is 'BRIEFINGS'.

'at' acts as a link.