Long boards behind fitter (7)
I believe the answer is:
readier
'fitter' is the definition.
(fit can mean ready or prepared)
'long boards behind' is the wordplay.
'long' becomes 'die' (to long for or to die for something is to crave it).
'boards' means one lot of letters goes inside another.
'behind' becomes 'rear' (both can mean one's bottom).
'die' placed inside 'rear' is 'READIER'.
(Other definitions for readier that I've seen before include "More prepared" , "More willing" , "more immediately available" .)