Hard black blocks used to be bad for fitting (4-5)

'fitting' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'hard black blocks used to be bad' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'black' becomes 'ba' (I can't justify this - if you can you should give a lot more credence to this answer).
'blocks' is an insertion indicator (the inside letters form a 'blockage').
'used to be' becomes 'was' (I've seen this before).
'bad' becomes 'sin'.
'h'+'ba'='hba'
'was'+'sin'='wassin'
'hba' going into 'wassin' is 'WASH-BASIN'.

'for' acts as a link.

Can you help me to learn more?