A small sailing boat put back in constantly (6)
I believe the answer is:
always
'constantly' is the definition.
(I've seen this before)
'a small sailing boat put back' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sailing boat' becomes 'yawl' (I've seen this before).
'put back' is a reversal indicator.
's'+'yawl'='syawl'
'syawl' written backwards gives 'lways'.
'a'+'lways'='ALWAYS'
'in' acts as a link.
(Other definitions for always that I've seen before include "Whatever happens!" , "On all occasions" , "Habitually" , "For good" , "whatever the circumstances" .)