In pen, tabloid girl with no heart is rambling (8)
I believe the answer is:
straggly
'is rambling' is the definition.
(I've seen this before)
'in pen tabloid girl with no heart' is the wordplay.
'in' is an insertion indicator.
'pen' becomes 'sty' (sty is a kind of pen**).
'tabloid' becomes 'rag' ('rag' can be a synonym of 'tabloid').
'with no heart' means to remove the middle letters.
'girl' with its middle removed is 'gl'.
'rag'+'gl'='raggl'
'sty' placed around 'raggl' is 'STRAGGLY'.
(Other definitions for straggly that I've seen before include "in disarray" , "untidy state" , "Spreading out in different directions" , "Irregularly untidy" , "rambling" .)