Count on getting extremely lucky (4)
I believe the answer is:
rely
'count' is the definition.
(I've seen this in another clue)
'on getting extremely lucky' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'getting' means one lot of letters go next to another.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'lucky' with its centre removed is 'ly'.
're'+'ly'='RELY'
(Other definitions for rely that I've seen before include "Have confidence or faith in" , "All the letters" , "City with cathedral" , "Bank (on someone)" , "Count" .)