Similar article penned by doctor with great enthusiasm (4,3)
I believe the answer is:
like mad
'with great enthusiasm' is the definition.
(I've seen this before)
'similar article penned by doctor' is the wordplay.
'similar' becomes 'like' ('like' can be a synonym of 'similar'**).
'article' becomes 'a' (indefinite article).
'penned by' is an insertion indicator.
'doctor' becomes 'md' ('MD' can be a synonym of 'doctor').
'a' put within 'md' is 'mad'.
'like'+'mad'='LIKE MAD'
(Other definitions for like mad that I've seen before include "With furious enthusiasm" , "Very quickly" , "With great intensity, energy, or enthusiasm" , "wildly" , "Energetically" .)