Chair seen in part of garden, mostly (6)
I believe the answer is:
rocker
'chair' is the definition.
(I know that rocker is a type of chair)
'part of garden mostly' is the wordplay.
'part of garden' becomes 'rockery' (I've seen this before).
'mostly' means to remove the last letter.
'rockery' with its final letter taken off is 'ROCKER'.
'seen in' acts as a link.
(Other definitions for rocker that I've seen before include "Someone who likes form of pop" , "1960s teenager" , "special chair" , "Opponent of mod" , "Mod's 1960s rival - curved support under chair legs" .)