Give away extremely useful key in seedy bar (7)
I believe the answer is:
divulge
'give away' is the definition.
(divulge can mean to give away information)
'extremely useful key in seedy bar' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'key' becomes 'g' (musical key).
'in' indicates putting letters inside.
'seedy bar' becomes 'dive' (as in a dive bar).
'useful' with its middle taken out is 'ul'.
'ul'+'g'='ulg'
'ulg' going within 'dive' is 'DIVULGE'.
(Other definitions for divulge that I've seen before include "Disclose, give away" , "Make known (a secret)" , "Make public" , "Reveal as with information" , "Make (secrets) known" .)