By the side of a pine (5)
I believe the answer is:
along
'by the side' is the definition.
(I've seen this in another clue)
'a pine' is the wordplay.
'pine' becomes 'long' (to pine for something is to long for it).
'a'+'long'='ALONG'
'of' is the link.
(Other definitions for along that I've seen before include "From one end to the other" , "(Going) in company (with others)" , "Lengthwise" , "Sea monster, opposite Charybdis" , "Lining" .)