Swimmer stuck between rocks laterally (8)
I believe the answer is:
sideways
'laterally' is the definition.
(I've seen this before)
'swimmer stuck between rocks' is the wordplay.
'swimmer' becomes 'ide' (type of fish, something that swims).
'stuck between' means one lot of letters goes inside another.
'rocks' becomes 'sways' (I've seen this in another clue).
'ide' going inside 'sways' is 'SIDEWAYS'.
(Other definitions for sideways that I've seen before include "Crabwise, laterally" , "Movement of crab" , "Walk this way, if you're a crab" , "Lateral movement" , "Crabwise, at an angle" .)