Planes that climb gradually (5)
I believe the answer is:
ramps
'planes' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe you can see an association between them that I don't see?
'climb gradually' is the wordplay.
'climb' becomes 'ramp' (ramping is a kind of climbing).
'gradually' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ramp'+'s'='RAMPS'
'that' is the link.
Can you help me to learn more?
(Other definitions for ramps that I've seen before include "Slopes to calm traffic" , "Gradients" , "Sloping passages" , "Slopings passages" , "Small road ridges for prams" .)