Compare to growth on rocks, say (5)
I believe the answer is:
liken
'compare' is the definition.
(I know that compare can be written as liken)
'growth on rocks say' is the wordplay.
'growth on rocks' becomes 'lichen' (I've seen this in another clue).
'say' shows a homophone (sound like).
'lichen' sounds like 'LIKEN'.
'to' is the link.
(Other definitions for liken that I've seen before include "Equate" , "Represent as similar" , "Point out the resemblance (to)" , "Compare" .)