Hard in small unfriendly place of learning (6)
I believe the answer is:
school
'place of learning' is the definition.
(I've seen this before)
'hard in small unfriendly' is the wordplay.
'hard' becomes 'h'.
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'unfriendly' becomes 'cool' ('cool' is associated in meaning with 'unfriendly').
's'+'cool'='scool'
'h' placed inside 'scool' is 'SCHOOL'.
(Other definitions for school that I've seen before include "a lot of swimmers" , "Forms" , "Educational establishment" , "Teach; group of fish" , "eg Harrow" .)