One in eight perhaps bank on Queen (5)
I believe the answer is:
rower
'one in eight perhaps' is the definition.
(eight is a type of rowing crew)
'bank on queen' is the wordplay.
'bank' becomes 'row' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'row'+'er'='ROWER'
(Other definitions for rower that I've seen before include "Person using 9" , "Might one argue" , "One in eight, perhaps" , "Oarsman/woman" , "Maybe one in eight" .)