Poles having trouble with garden pest (5)
I believe the answer is:
snail
'garden pest' is the definition.
(I've seen this before)
'poles having trouble' is the wordplay.
'poles' becomes 'sn' (North and South poles).
'having' says to put letters next to each other.
'trouble' becomes 'ail' (synonyms).
'sn'+'ail'='SNAIL'
'with' acts as a link.
(Other definitions for snail that I've seen before include "trailer" , "Proverbially slow moving mollusc" , "Escargot" , "10 ac mollusc" , "Land mollusc" .)