Mark keeps fish in motorcycle attachment (7)
I believe the answer is:
sidecar
'motorcycle attachment' is the definition.
(I've seen this before)
'mark keeps fish' is the wordplay.
'mark' becomes 'scar' (synonyms).
'keeps' means one lot of letters goes inside another.
'fish' becomes 'ide' (I've seen this before).
'scar' enclosing 'ide' is 'SIDECAR'.
'in' acts as a link.
(Other definitions for sidecar that I've seen before include "Appendage to motor cycle" , "passenger compartment" , "Low-slung two-wheeler" , "that biker may take with him" , "Alcoholic drink" .)