Fairly big line in front of garish building (7)
I believe the answer is:
largish
'fairly big' is the definition.
(I've seen this before)
'line in front of garish building' is the wordplay.
'line' becomes 'l'.
'in front of' says to put letters next to each other.
'building' is an anagram indicator.
'garish' is an anagram of 'argish'.
'l'+'argish'='LARGISH'
(Other definitions for largish that I've seen before include "quite generous" , "sizeable proportions" , "Comfortable shoe" , "Of considerable extent" , "Somewhat hefty" .)