Fertile spot, like one adopted by map-makers (5)
I believe the answer is:
oasis
'fertile spot' is the definition.
(I've seen this before)
'like one adopted by map-makers' is the wordplay.
'like' becomes 'as'.
'one' becomes 'i' (Roman numeral).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'map-makers' becomes 'os' (Ordnance Survey).
'as'+'i'='asi'
'asi' inserted into 'os' is 'OASIS'.
(Other definitions for oasis that I've seen before include "Fertile patch in a desert" , "Fertile tract in a desert" , "Desert refuge" , "retreat" , "Peaceful area away from trouble" .)