Finally directing, guide well (6)
I believe the answer is:
gusher
'well' is the definition.
(I know that gusher is a type of oil well)
'finally directing guide' is the wordplay.
'finally' indicates one should take the final letters.
'guide' becomes 'usher' (an usher guides people).
The last letter of 'directing' is 'g'.
'g'+'usher'='GUSHER'
(Other definitions for gusher that I've seen before include "one's over-effusive" , "An active oil well or excessive talker" , "effusive type" , "Effusive person or oil well in action" , "one exuding enthusiasm" .)