Best I start to cram for subject (5)
I believe the answer is:
topic
'for subject' is the definition.
(as in a topic for discussion)
'best i start to cram' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'start to' indicates taking the first letters.
The first letter of 'cram' is 'c'.
'top'+'i'+'c'='TOPIC'
(Other definitions for topic that I've seen before include "'Theme, subject (5)'" , "Talking point" , "Question" , "Subject (eg, of text)" , "might this be hot?" .)